Computer Won't Turn On: What to Check First (2026)

By LK Wood IV · Published 2026-09-28 · Updated 2026-10-13 · ~10 min read · St. Louis County, MO

Two different faults hide behind “my computer won’t turn on.” In one, nothing happens at all when you press the button. In the other, the fans kick for a second and everything stops. They share the first few minutes of checks and then split, so it pays to know which one you have before you open anything.

This page is for a desktop that will not start. If yours powers on with the fans running and the screen stays black, that is a display or POST fault, and my graphics card not detected guide starts from there.

No power, no POST, no boot, or no video?

Dell’s current startup diagnostic separates four states. No power means no fan, light, or diagnostic LED. No POST means the machine has power but never completes its hardware checks. No boot means POST completed but Windows does not load. No video means the computer runs but the display gets no picture.

This page covers the first state and the closely related case where the fans twitch and stop. A search such as “CPU won’t turn on” usually describes the whole tower, because the processor has no separate power button. If fans and lights stay on, do not test the case switch or paperclip the power supply. Move to the No Signal guide and read the motherboard’s CPU, DRAM, VGA, or BOOT light instead.

If the PC has power only in the sense that a motherboard standby light glows while the fans stay still, continue here. Standby power confirms that some power reached the board; it does not prove the case button, main PSU outputs, or motherboard start circuit works.

Match what the PC does to where the fault is

The last row is easy to miss. A press normally wakes a sleeping PC, and on the board in my desktop ASUS’s manual says the power LED blinks in sleep. A machine that ignores the press may be hung while still running, and for a running system the ACPI specification defines holding the power button for four seconds or longer as an override that sends it to soft-off, so the next press is a clean start.

CPU failure symptoms: what does the evidence prove?

Check compatibility first.

A powered PC that cannot finish POST may illuminate a CPU diagnostic light, but that is an investigation point rather than a replacement order. ASUS’s no-boot procedure checks the exact CPU against the motherboard support list and required BIOS version. It also checks power and memory before the display path. Use the manual for your board; an unsupported firmware version can matter even when the socket fits.

Record which diagnostic light remains on and whether the system ever displayed its firmware screen. Power off and disconnect power before inspecting user-serviceable connections. Do not handle socket pins or swap processors as a first test. If the manual’s checks do not isolate the fault, a repair shop can compare compatible known-good parts. Crashes, heat, fan motion or a CPU light alone do not establish a failed processor.

If a laptop screen blinks while Windows keeps running, start with the screen-flicker diagnosis, including its Task Manager comparison. If a GPU lights up but no picture appears, use the no-signal checks; lighting does not prove successful POST or a working display path.

Outside the case first

None of this needs tools. All of it together takes about a minute.

1. Plug a lamp or a phone charger into the same wall outlet. Intel’s troubleshooting guide starts here, with a device you know works.
2. Take the power strip or UPS out of the chain and plug the PC straight into the wall.
3. Check the rocker switch on the back of the power supply. I is on and O is off, and a moved tower can knock it to O.
4. Reseat the power cord at both ends. Intel suggests trying a spare, including a monitor cord, only when its connector, wire gauge and current rating meet the PC’s requirements. Check the markings; a cord that fits is not enough.
5. Unplug everything else from the back: USB devices, display cables, the lot. Intel notes that USB devices and display cables occasionally cause power problems.

Then drain it. Pull the cord from the power supply, hold the case power button for 15 to 20 seconds, plug back in, set the rear switch to I and press power once. That hold is Dell’s desktop step for discharging residual power, and it costs nothing.

Look for a light on the motherboard

With the supply plugged in and its switch on, look at the board through the side panel before you press anything. Any lit LED there is good news.

The ATX +5VSB standby output can remain active while the main outputs are off. A motherboard light shows that some power reached the board; it does not establish voltage quality, all required outputs or the start circuit. The case button and its lead are among the things to check, not a diagnosis from the LED alone.

The board in my desktop is an ASUS ROG STRIX B650-A Gaming WiFi, and ASUS’s manual for it names two lights worth knowing. The case power LED on that board’s header “lights up when the system is connected to a power source”, so a glowing power LED with the PC off means standby is there. A second LED, marked PLUG_8PIN_PWR, “lights up to indicate that the 8-pin power plug is not connected.” If that one is lit, switch off and disconnect AC before checking the CPU power connection against the manual.

No light proves nothing. Plenty of boards have no standby LED, so carry on either way.

Start it with a screwdriver

Find the manual first. This optional test bypasses the case switch for one startup attempt, so it depends on identifying the exact power-switch pins and following the system maker’s service rules, particularly with a prebuilt or proprietary front-panel connector. A documented onboard start button is easier to use. Unclear pins mean stop.

The case power button does very little. It connects two pins on the motherboard’s front-panel header for a moment. The board then pulls the supply’s PS_ON# line low, which Intel’s ATX guide defines as the signal “that allows a motherboard to remotely control the power supply”, and the supply turns on its main outputs. You can make the button’s momentary connection yourself.

1. Switch the PSU off and disconnect its AC cord. Follow the manual’s discharge/wait instructions before opening the case or moving a connector.
2. Find the exact power-switch pair in the motherboard manual. On my B650-A, ASUS labels it PWRBTN# and GND. Do not apply that position to another board or use a clear-CMOS header.
3. With AC still disconnected, remove only the case power-switch lead. Leave required motherboard/CPU power connected and the CPU cooler installed, connected and working. Keep loose leads clear.
4. Reconnect AC and switch on only when ready for the manual’s momentary switch-pin test. Briefly bridge only the documented pair, or press the documented onboard button. Do not move any connector while powered.
5. After observing the attempt, switch off and disconnect AC before removing, reseating or moving any lead.

Record the result rather than assigning a failed part:

• It starts from the documented pair but not the case button. Inspect the case switch, lead and seating with AC disconnected. This narrows that path; it does not certify the PSU or motherboard under load.
• Nothing happens. Check the documented setup and required connections. The result alone does not clear the button or condemn the supply, board or CPU; continue with the power path.
• A twitch, then off. Stop repeating the attempt and read twitch and die.

The button or its lead

The repair costs nothing if the case has a reset button. Intel’s guide says the reset and power switches “operate in the same way”, so, if your exact case and board use compatible momentary switches, the reset button can substitute. Switch off and disconnect AC before moving its lead onto the documented power-switch pair; confirm both manuals first. You lose the reset function. On my B650-A the reset pair sits in the same header, labeled RSTCON# and Ground.

Look at the lead itself before you settle for that. A plug that slid half off one pin, or a wire broken where it enters the plug, gives exactly this symptom, and reseating or replacing that lead is the whole repair. A button that feels stuck or gritty is a reason to inspect the switch mechanism, and for a failed case button Intel’s advice is to ask the case maker for the long-term fix.

Follow the power back to the PSU

If the documented switch test did not start the machine, required power connections and the supply remain among the unresolved checks. Unplug the PC from the wall before you touch a cable.

Reseat both motherboard power cables, the wide 24-pin and the 8-pin CPU cable, until each latch clicks. On a modular unit, reseat both ends. Some boards carry a second CPU power socket — a 4-pin or another 8-pin — and ASUS’s no-power FAQ recommends filling every one the board has, while the manual for my B650-A asks for the 8-pin alone or the 8-pin and the 4-pin together.

The 24-pin matters more than it looks, because the power-on signal travels through it. On the standard ATX connector, PS_ON# is pin 16. Identify it from the exact documented connector view, never wire color; all-black cables and proprietary connectors make color shortcuts unreliable. Intel’s design guide says that when PS_ON# is open-circuited the main outputs stay at zero, so a pin that never makes contact leaves the board with no way to start the supply.

Test the power supply on its own

Keep the PSU enclosure closed. Use the supply maker’s supported procedure and supplied jumper or tester, if available; the PSU failure guide explains what startup and simple tester results can establish, and why neither replaces evidence under load.

Corsair’s test procedure, checked October 13, 2026, shows the connector orientation and supported pins. Read its zero-RPM and tester notes too: newer supplies can produce readings that an older tester labels incorrectly. Match the procedure to the unit. Wire color is insufficient, and a standard ATX test cannot be assumed to fit a proprietary connector.

Disconnect AC before preparing the test. Attach or remove the documented jumper or tester only while disconnected, and reconnect power only for the observation the manufacturer describes. Disconnect again before changing anything.

No applicable procedure? Leave the result unknown and ask the maker. There is no need for live multimeter probing here.

A still fan can be normal. Zero-RPM and fanless designs make that observation particularly easy to misread, while an OEM self-test button has whatever meaning its model’s service instructions specify. Ask the manufacturer to interpret an unclear result before replacing the supply.

A compatible borrowed PSU can help. Use its approved cable set. Record each reseated connection, because stable operation with the replacement implicates the old configuration without separating the original supply from its cables and connections. If the machine still will not start, required firmware/CPU support, memory, board, cooling and power connections remain in the investigation.

If it twitches and dies

A twitch shows that startup did not continue. A tripped protection circuit is one possible explanation; the symptom alone does not identify the fault. Corsair’s explainer says short-circuit protection shuts the supply down “should the 3.3V, 5V, and 12V rails short to any other rail, or to ground”, and its over-power protection trips at 115 to 135 percent of rated output.

After a trip, Intel’s ATX guide says the supply returns to normal only once the fault has been removed and its on signal has been off for at least one second. Do not repeatedly provoke shutdowns. Switch off and disconnect AC before inspection; a recurring trip needs its cause resolved.

Look for a short first. Intel names two: a motherboard screwed to the case without the standoffs it needs, and stray Molex pins touching the chassis. After a new build, those two and anything metal loose in the case are where I would look.

Disconnect AC. Follow the exact motherboard manual for a minimum-configuration test and confirm CPU/firmware support. The CPU cooler stays installed with its power connected. Where the manual permits it, remove the graphics card and drives, disconnect external devices, and leave one RAM module in the prescribed slot (DIMM_A2 on my B650-A).

If it now stays on, add one part back. Switch off and disconnect AC before each change. Record when the symptom returns, including the cable and connection you used, because each added device also changes the load on the supply.

You do not need a picture for this. The only question is whether it stays on with the fans running.

Still twitching? Stop repeated starts. Inspection alone has not excluded every short or compatibility problem, even in the documented minimum configuration with CPU cooling retained. A compatible borrowed supply can be the next comparison if the maker’s instructions support it.

On a laptop

A laptop gives you less to reach.

• Try a known-good charger only if the manufacturer confirms the required voltage, polarity, connector and power rating or USB-C PD profile. Physical fit alone is insufficient. Check its indicator using the model’s instructions.
• If the battery is user-removable and the model’s instructions permit it, disconnect the charger before removing it and follow the stated AC-only test. Do not open or handle a swollen or damaged battery.
• Do a power reset. HP’s version is to shut down, disconnect the charger and any nonessential devices, hold the power button for about 15 seconds, then reconnect and start.
• Only models explicitly listed by Dell support its real-time clock reset; save/recover required firmware settings before using that model-specific procedure: with the charger connected, hold power for 30 to 35 seconds until the power light blinks three times. Dell says it aborts under 25 seconds or over 40, and that it resets the BIOS settings, date and time.

Past that, a laptop showing no sign of power is a repair or warranty job.

What not to do

• Never plug modular cables from your old power supply into a new one. Corsair’s guidance is to “never try to use one brand’s cables with another brand’s PSU”, because each maker wires the PSU end its own way, and even within one brand the cable type has to match.
• Do not short pins you have not identified. On my B650-A, ASUS warns that shorting or jumpering the clear-CMOS header, CLRTC, outside of a CMOS clear “will cause system boot failure.” Read the label before the screwdriver touches anything.
• Keep the power supply’s case closed. It can hold a dangerous charge after it is unplugged, which my PSU guide covers in its safety section.
• A pop, a spark or a burning smell ends the diagnosis. Unplug the PC and leave it off.
• Ignore generic beep-code charts. Intel’s guide points out that every motherboard maker uses its own codes, so read your board’s manual.

When to stop and send it back

• An unclear or failed manufacturer-supported PSU test goes to the maker with the exact model, setup and observation. Do not condemn a zero-RPM, fanless or unsupported unit from fan motion alone.
• If the documented minimum configuration still will not start, take the model/firmware, cooling, cable-set and test record to the system maker or a technician with compatible parts. A failed start is not enough to choose between the board and CPU.
• For a prebuilt, ask the maker which user-serviceable checks its service instructions permit before opening the case.