The PC hums, the keyboard lights blink, and the monitor insists there is no signal — or shows nothing at all. Before the computer takes the blame, interrogate the picture path: the monitor's selected input, the cable, and which socket on the PC that cable is plugged into. This page is those three checks, plus how to prove whether the PC secretly booted fine all along.
A black screen is evidence about the picture, not about the computer. Those are different machines that happen to share a desk. Modern monitors have several inputs and no loyalty — they show whichever input is selected, and if your cable sits in a different one, the PC can boot perfectly into a void. Add a cable that fails silently and a PC that offers two possible video sockets, and you have the three usual suspects. All three are free to test.
Every monitor has a way to choose its input: a dedicated Source/Input button or a menu option, cycling through HDMI 1, HDMI 2, DisplayPort, VGA. Children press it, cleaning cloths press it, and some monitors switch it themselves after a power cut. Find the button, open the input list, and select the input your cable actually uses. If your monitor offers Auto Source, turning it on prevents a repeat. This single check ends a large share of no-signal house calls, and it costs one minute.
Many desktops offer video sockets in two places: on the motherboard's back panel, and lower down on the graphics card. When a graphics card is fitted, most boards switch the motherboard's video output off — a cable plugged there receives nothing, forever, while the PC boots happily. Look at the back of the PC: if a graphics card is present (a horizontal card with its own HDMI/DisplayPort sockets), the cable belongs in the card. This mistake is at its commonest right after a PC is moved or a card is newly installed.
HDMI and DisplayPort cables fail quietly — a bent pin, a tired connector, a cable that worked for years and stopped mid-week. Test with any other cable, even a short one borrowed from a TV, and try the other port on both ends if available. Push connectors fully home; DisplayPort plugs have a latch that needs a squeeze to release and a firm push to seat. A picture appearing on the test cable is a complete diagnosis: the PC was never ill.
While the screen stays black, gather witnesses. Tap Num Lock: if its light toggles, the machine is awake and processing. Listen for the Windows start sound. Watch the disk activity light settle down as it does after a normal boot. If the evidence says the PC booted, your problem is purely the picture path and the deep hardware checks can wait. If nothing responds at all, you are in the fans-but-no-display family from the split page — the RAM reseat is your next honest move.
The final free test: connect the PC to any other display — a TV with the right cable counts. A picture on the TV acquits the PC and convicts something in the original chain: that monitor, its input setting, or its cable. The reverse also holds — a monitor that shows its own settings menu brightly when you press its buttons has a working panel, narrowing the fault to signal, not screen. Diagnose with swaps, and the guilty part names itself.
In the video, a black-screen PC gets these same questions in this order. The monitor's confession usually arrives early.
▶️ Watch Full VideoYes. Some monitors auto-switch after a power cut or when a second device wakes, and a few models reset their source when fully unplugged. Cleaning the screen also presses more buttons than people realise. It takes one minute to check the input list, which is why technicians check it even when nobody admits to touching anything.
Because most boards disable the built-in video output when a separate graphics card is detected, so the processor's graphics go unused and that port stays dark. It is normal behaviour, not a fault. Plug the monitor into the graphics card's sockets — the horizontal card low on the back panel — and the picture returns.
Use the witnesses. If the Num Lock or Caps Lock light toggles when tapped, the machine is awake and running. Windows start sounds and the disk activity light settling into its normal pattern say the same. A PC that boots but shows nothing has a picture-path problem; a PC with a lifeless keyboard has a start-up problem, and they get different checks.