PC won't start:
what it's telling you

When not a single light comes on and no fan spins, the computer isn't getting power or the power supply isn't delivering it. Before thinking of a fault, rule out the simple things: test the socket with another appliance, take the extension lead out of the equation, check the cable is firmly seated at both ends and, on a desktop, that the switch on the back of the power supply is in the on position. On a laptop, plug in the charger and see whether its indicator light comes on; if the battery was completely flat, let it charge for a while before trying again. If it still shows no sign of life after all that, the most likely culprits are the power supply, the charger, the button or the motherboard, and there's no safe home check for those: opening a power supply is not an option, because its capacitors can hold a charge even when unplugged.

It means the computer is getting power, but something isn't completing the start-up or the picture isn't reaching the monitor. First rule out the silliest cause, which is also one of the most common: the monitor is off, the wrong video input is selected in the monitor's menu, a cable is loose or, on desktops with a graphics card, the cable is plugged into the motherboard's output instead of the card's. Trying another monitor, a TV or a different cable clears this up in minutes. If the screen is still black with all that ruled out, the usual suspects are the memory, the graphics card, the power supply or the motherboard. Bear in mind too that some recent machines take much longer than usual on the first boot after changing the memory or updating the BIOS, because they train the memory before showing a picture: give it a couple of minutes before forcing it off.

They're the computer itself telling you which part failed the initial check it runs before loading the system. The trouble is that the code isn't universal: each motherboard or BIOS manufacturer uses its own combinations of long and short beeps, and they even change between generations within the same brand. That's why a generic table found online can send you to the wrong part. What helps is noting the exact pattern (how many beeps, long or short, whether they repeat) and looking it up in the manual for your motherboard or computer model. Many modern motherboards have no speaker and instead have small labelled lights that stay on at the stage that fails, and some laptops flash the power or caps lock light in a specific pattern. Whatever form it takes, write it down: it saves us diagnosis time.

Not necessarily, but it's one of the possibilities and the one that matters most to rule out. If the manufacturer's logo appears, the basic hardware works: the fault comes afterwards, when trying to load the system. The usual causes are an update that was interrupted halfway, damaged boot files, a USB stick or external drive plugged in that the computer is trying to boot from, overheating or, indeed, a disk that's starting to fail. First disconnect everything that isn't essential and read calmly whether any message appears before the restart; a photo with your phone helps. If you hear clicks, squeaks or the disk spinning up and stopping, stop trying right then: every restart in the loop is one more start-up of a disk that may be damaged, and that's exactly what makes recovering its contents harder.

Let it finish a full cycle without interrupting it, because sometimes it fixes the problem by itself. If it comes back to the same point, Windows offers an advanced options menu with startup repair, system restore or uninstalling the latest update, and on a machine with a recent backup, trying those options is reasonable. What you shouldn't do blindly is choose «Reset this PC», because depending on the option you tick it can remove your files, or run disk checks that write to the disk to «fix» it. If the loop is caused by a failing disk, those tools keep it working for a long time and can alter exactly the area where your documents were. So the key question before touching anything is whether you have a backup of what matters. If you don't, switch the computer off and bring it in: first the contents are secured, then the system is repaired.

On a desktop computer, yes, if you're comfortable opening the case; it's one of the checks that most often fixes a black screen with the fans spinning. Switch off, unplug the power cable, touch an unpainted metal part of the case to discharge static electricity, open the clips on the module, take it out and push it back in straight and firmly until the clips close by themselves. Hold the modules by the edges, without touching the gold contacts, and don't force anything: if it won't go in, it's the wrong way round. A laptop is another story: on many models the memory is soldered to the board and on others you have to take a lot apart to reach it, with fragile clips and ribbon cables along the way. There, the risk isn't worth it. And if you don't fancy opening the desktop either, that's fine: it's a five-minute check in the workshop.

It can fix a damaged system, but it's the last option, not the first, and never before your data is safe. Reinstalling means writing many gigabytes to the same disk your documents are on, and depending on how it's done it can delete them outright or overwrite the area where they were. What's more, if the real cause was a disk starting to fail, the reinstall will go wrong halfway through or the new system won't last long, and you'll have used up hours of that disk's working life at the worst possible moment. The right order is the reverse: first check the disk's health and copy what matters, then decide whether the system can be fixed or is better installed from scratch. If by then you have a verified backup and the disk is healthy, reinstalling is a clean and perfectly valid solution; if you don't, it's the step that most often turns a scare into a loss.

It depends on which part has failed and the state of the rest of the machine, so it can't be answered before the diagnosis. A cable, a charger, a power supply or a disk are contained repairs that usually give a computer that's otherwise healthy its working life back. A motherboard in a laptop several years old, or several parts at once in a machine that no longer gets system updates, is a different conversation, and it's worth comparing before deciding. What's independent of that decision is your data: even if the computer isn't worth repairing, what's on its disk can almost always be taken off and moved to the new computer if the disk is fine. That's why the first thing is to know what has really failed. With that in front of us, we tell you what each option would cost, and if the conclusion is that it isn't worth repairing, we'll tell you that too.