PC turns on but no display — fans spinning, lights glowing, and a monitor stubbornly reporting “no signal”. It’s the fault that feels catastrophic and usually isn’t: power flowing proves almost nothing about whether the computer is actually starting, and the real cause is more often a cable, an input setting or a loose stick of RAM than a dead part. Here’s the exact order we check things at the bench — humble stuff first, because the humble stuff wins embarrassingly often.
Brand-new build showing nothing? That’s a different diagnostic tree (memory training, front-panel wiring, BIOS support) — our new PC build not booting guide is written for exactly that. This guide is for a PC that used to work.
First: is the PC alive behind the black screen?
One test splits this whole problem in half. Press Caps Lock on your keyboard: if the little light toggles on and off, the PC has completed its power-on self-test and is running blind — the fault is somewhere between the graphics output and your monitor. You can double-check with Win + Ctrl + Shift + B, which restarts the graphics driver and chirps if Windows is alive, or just listen for the login sound.
If Caps Lock is dead, the machine isn’t POSTing — skip to the RAM section, and check your motherboard’s debug LEDs (small lights labelled CPU / DRAM / VGA / BOOT near the 24-pin connector): whichever stays lit is naming the suspect for you. Older boards without LEDs often speak in beep codes instead — a repeating pattern of beeps at power-on is the same message in Morse; count the pattern and check it against your board manual.
The display chain: cable, input, port
Check the monitor’s input source — monitors love sitting on HDMI 2 while you’re plugged into HDMI 1. Cycle through inputs with the monitor’s buttons.
Confirm the cable is in the graphics card, not the motherboard’s video outputs — after moving or cleaning a PC this mix-up is a bench classic.
Reseat the cable at both ends, then try a different cable, and a different port on the card (DisplayPort ↔ HDMI).
Prove the monitor: plug a laptop or console into it. Screen works elsewhere — the monitor’s off the suspect list.
The RAM reseat (the fix that wins most often)
If the PC isn’t POSTing — dead Caps Lock, DRAM debug light on — reseat the memory before anything else. Power off at the wall, hold the power button five seconds to drain, then release both clips on each stick, pull it fully out, and push it firmly back until both clips click on their own. Still nothing? Try one stick only, in the slot your motherboard manual lists first (usually the second slot from the CPU), and rotate sticks and slots to find a bad one. Transported, knocked or recently-cleaned PCs make this the single most common no-display fix we perform — and it’s free.
Graphics card checks
VGA debug light on, or POST completes but no output anywhere? Reseat the graphics card itself: release the slot’s latch, remove, and push back until the latch clicks — then confirm every PCIe power connector is fully home (12VHPWR connectors especially reward a firm final push). The gold isolation test: if your CPU has integrated graphics, pull the card entirely and run from the motherboard’s video output. Desktop appears on integrated — the card, its power feed or its slot is your fault domain. One more: if the card hangs off a riser cable in a vertical mount, that riser just became suspect number one — our PCIe guide explains why risers and high link speeds fall out.
Clear CMOS: the settings reset
No display immediately after a BIOS change, an XMP/EXPO enable, or a failed overclock? The board may be trying to start with settings the hardware can’t run. Clearing CMOS resets it: power off and unplug, then either short the CLR_CMOS pins with a screwdriver for ten seconds, press the dedicated button (many boards have one on the rear panel), or pop the coin battery out for a minute. Settings return to safe defaults, and displays frequently return with them.
Works in BIOS, black in Windows?
Seeing the motherboard logo and BIOS screens, but blackness once Windows takes over, flips the diagnosis entirely: hardware fine, graphics driver or display mode broken — Windows is outputting something your monitor or cable can’t show. Boot into Safe Mode (it uses a basic video mode), remove the driver completely with DDU, reinstall fresh, and check you haven’t set a refresh rate beyond what the cable supports. This is also the pattern after a driver update goes wrong — and if the machine has been restarting itself as well, our random restarts guide picks up that thread.
When it’s a bench job
If you’ve proven the monitor, reseated RAM and GPU, tried integrated graphics and cleared CMOS — and Caps Lock still won’t blink — you’re into territory where diagnosis needs known-good spare parts: a test PSU, a test CPU, a spare board. That’s not a failure of effort; it’s just what proving a dead motherboard requires, and it’s exactly what a bench keeps on the shelf. Bring it to us in Harrow and we’ll isolate the part and quote before touching anything — no fix, no fee.
The bottom line: Caps Lock first — it tells you which half of the problem you have. Then cable and input, then the free reseats, then CMOS. The expensive explanations are the rare ones; the two-minute checks solve most black screens we see.
Screen still black? Tell us what the debug lights show and what you’ve tried — we diagnose no-display PCs every week in Harrow, collection available across North West London.
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