The short answer
Fans spinning does not mean the computer is running. Reseat the memory first, because it is the most common cause and costs nothing. Then check the monitor is on the right input and connected to the graphics card rather than the motherboard. Listen for beeps and look for diagnostic lights, which name the failing component on most modern motherboards.
A computer that powers up but shows nothing has failed during the power-on self test, before it ever reaches the operating system. That is useful, because it means the problem is in a small set of components: power, memory, graphics, processor or motherboard, roughly in that order of likelihood.
Step 1: confirm it is actually the computer
Check the monitor's input selection
Monitors do not always switch automatically, and a monitor on the wrong input shows the same black screen as a dead computer.
Check which port the cable is in
If the computer has a graphics card, the display must be connected to the card, not the motherboard’s port. This catches people after moving a machine.
Try another cable and another monitor
Cheap tests that eliminate two possibilities.
Look for signs of life
Keyboard lights, a drive light, a beep at power-on. A machine that lights the keyboard’s caps lock key has at least started the self test.
Step 2: read the diagnostics
Most motherboards made in the last decade have either a row of small LEDs labelled CPU, DRAM, VGA and BOOT, or a two-digit code display. The LED that stays lit names the stage that failed, which is the most valuable diagnostic information available and one most people do not know exists.
Older boards use beep codes through a small internal speaker, if one is fitted. The manual lists what each pattern means, and patterns differ by BIOS manufacturer.
If the DRAM light stays on, go straight to the memory steps below. If the VGA light stays on, the graphics card is not being detected. If the CPU light stays on, check the processor power connector, which is a separate cable from the main one and is frequently left unplugged after work inside the case.
Step 3: memory
Memory is the most common cause of this symptom, and reseating it is free.
- Power off, unplug, and hold the power button for five seconds to discharge.
- Remove each memory module and refit it firmly until both clips close. They require more force than people expect.
- Try booting with a single module, in the slot the manual specifies for one stick.
- If that fails, try the same module in a different slot, then the other module.
- Clean the contacts gently with a pencil eraser if the machine is old or dusty, then remove residue.
Testing one module at a time is what identifies a failed stick or a failed slot, and it takes a few minutes.
Step 4: graphics
- Reseat the graphics card, making sure the retaining clip engages.
- Check both supplementary power connectors are fully seated. These carry a lot of current and a partially seated connector causes exactly this symptom.
- If the processor has integrated graphics, remove the card entirely and connect the monitor to the motherboard. If the machine boots, the card or its slot is the fault.
- Try the card in a different slot if the board has one.
Step 5: power
Fans spinning does not prove the power supply is healthy, because fans start on a fraction of the required output. Check:
- The 8-pin processor power connector at the top of the board, which is easy to miss.
- The 24-pin main connector, fully seated with its clip engaged.
- All connectors at the power supply end on a modular unit, which is where they are most often loose.
- Whether the machine was recently upgraded with a component that draws more power.
Step 6: clear the BIOS
A failed overclock or a corrupted setting prevents booting and looks like hardware failure. Clearing the BIOS resets everything to defaults:
- Power off and unplug.
- Move the clear CMOS jumper for a few seconds, or press the clear button if the board has one, or remove the coin cell battery for five minutes.
- Reassemble and try again.
This is worth doing early if the problem appeared after changing settings, updating firmware or installing new hardware.
Work with the machine unplugged, and touch a bare metal part of the case before handling components. Static damage is silent and permanent, and it is the one risk in an otherwise safe process.
Step 7: minimum configuration
If nothing has worked, strip the machine to the minimum that can boot: motherboard, processor and cooler, one memory module, and either integrated graphics or one card. Disconnect all drives, extra fans, front panel USB and anything else. If it posts, add components back one at a time until it fails, which identifies the culprit directly.
This sounds laborious and is usually faster than guessing, and it costs nothing.
What it probably is
In rough order: memory not seated, monitor on the wrong input or cable, graphics card power connector loose, processor power connector unplugged, failed power supply, cleared BIOS needed, failed memory module, failed graphics card, failed motherboard. Working down that list solves the great majority without buying anything.
The fans spin, so the power supply must be fine, right?
No. Fans start on very little power. A power supply can spin fans while failing to deliver stable voltage to the processor and memory, which produces exactly this symptom.
What do the motherboard LEDs mean?
Each represents a stage of the self test: processor, memory, graphics and boot device. The one that stays lit names the stage that failed, which points directly at the component to check.
Why does reseating the RAM fix it so often?
Memory modules work loose from thermal cycling and vibration, and a module that is slightly out of contact fails the self test completely. It is the single most productive thing to try first.
Should I clear the BIOS?
Yes, early, if the problem appeared after changing settings or hardware. It reverts everything to defaults and takes a few minutes, and it costs nothing but your saved preferences.
Could it be the processor?
Possible and uncommon. Processors rarely fail outright. Check the processor power connector and the cooler mounting before suspecting the chip itself.
The order
Monitor input and cable, then look at the motherboard’s diagnostic lights. Reseat memory, one module at a time. Reseat the graphics card and its power connectors, and try integrated graphics if available. Check the processor power connector. Clear the BIOS. Then strip to minimum configuration and add parts back. That sequence identifies nearly every case of a machine that powers on and shows nothing.
