The computer will not turn on

Nothing happens when you press the button. Before assuming the worst, work through this in order. A good proportion of these turn out to be something trivial.

First, what exactly happens?

The answer changes everything, so be precise.

Absolutely nothing. No lights, no fan noise, no sound. This is a power problem.

Lights and fans, but no picture. The computer is running. This is a display problem, and it is much better news.

It starts and then stops after a few seconds. Usually power supply or overheating.

It starts, shows the manufacturer logo, then fails. The hardware is fine and the problem is with Windows or the drive.

No lights at all

Check the power at the wall. Try a different socket, and test that socket with a lamp. Power boards fail more often than computers do, and their switches wear out.

Check both ends of the cable. The end that plugs into the computer works loose surprisingly easily, particularly on desktops that get moved.

On a laptop, this is usually the charger. Look for a light on the charger brick or on the laptop when plugged in. No light means the charger, the cable or the socket.

Laptop chargers fail constantly, almost always at the point where the cable meets the plug, from being wrapped tightly. Wiggle it gently while watching for the light to flicker. If it flickers, that is your answer, and a replacement charger is far cheaper than a repair.

On a desktop, check the switch on the back of the power supply. There is often a small I/O rocker switch beside the cable that gets knocked.

Try holding the power button for 30 seconds with the power disconnected, then reconnect and try again. This discharges residual power and genuinely resolves a fair number of cases.

Lights and fans, but a blank screen

Good news: the computer is running.

On a desktop, check the monitor. Is it on? Is it set to the right input? Monitors have an input button, and they get switched to HDMI 2 when nothing is connected there. Is the cable firmly in at both ends?

Try a different cable, and a different port. Cables fail, and so do individual ports.

On a laptop, look very closely at the screen in a bright room. If you can faintly see the desktop, the backlight has failed rather than the screen. That is a repair, not a replacement.

Try an external monitor on a laptop. If that works, the fault is the screen or its cable, and everything else is fine.

It turns on and off again

Overheating is the common cause, particularly in an older desktop full of dust. Vents blocked, fans clogged, and the machine shuts down to protect itself. Compressed air and ten minutes of cleaning fixes many of these.

A failing power supply in a desktop. These do not fail cleanly; they usually go through a period of odd behaviour first.

On a laptop, a swollen battery. If the case has started to bulge or the trackpad has lifted, stop using it and do not charge it. Swollen lithium batteries are a genuine fire risk and want proper disposal, not the bin.

It starts but Windows does not

The hardware is fine, which is the important part. Your files are almost certainly still there.

Turn it off and on three times, interrupting the boot each time. Windows will offer its recovery options.

Try Safe Mode from those recovery options. If it starts in Safe Mode, the problem is software: a driver, an update, or something recently installed.

If it mentions no boot device or no operating system, the drive may have failed. This is where a backup stops being theoretical.

What not to do

Do not keep pressing the power button repeatedly. It will not help.

Do not open a laptop unless you know what you are doing, particularly if a swollen battery is suspected.

Do not pay for phone “diagnostics” from anyone who rang you. Nobody legitimate rings about your computer. See the fake tech support guide.

Do not let anyone wipe the drive before recovering your files, if there is anything on it you have not backed up. Say this explicitly to any repairer.

Is it worth fixing?

Usually yes: a charger, a cable, a power supply, dust and overheating, a failed screen on an otherwise good laptop.

Usually not: a failed motherboard on a machine over five years old, a laptop where the repair approaches half the cost of a replacement, or a machine that can no longer run a supported version of Windows.

Always separate the data question from the repair question. Even if the computer is finished, the drive is usually fine and your files can be recovered from it. Do not let a machine go to the tip until you are certain there is nothing on it you want.

Before you replace it

It is worth having somebody look before you spend money. A meaningful proportion of computers written off as dead turn out to need a $60 charger or half an hour of dust removal.

That is a quick and honest thing to check, and if the answer is genuinely that it is finished, you will be told that plainly rather than sold a repair. Get in touch.