A computer that will not start feels like the worst case, and it usually is not. The cause is often the power supply, the cable, the screen or a stuck state, all of which are cheap or free to fix. Work through the checks in order, and stop when it starts.
First, which kind of "will not turn on"?
- Nothing at all: no lights, no fans, no sound. Read the power section.
- Lights or fans, but a black screen. Read the display section.
- It starts, shows the maker's logo, then stops, restarts or shows an error. Read the start-up section.
Nothing at all: power
A desktop. Check the cable at both ends, the switch on the back of the power supply (it should be on the "I" side), and the wall socket, by plugging in a lamp. Try a different cable if you have one; the cable is the same kind as a kettle lead and fails more often than people expect. If there is still nothing, the power supply is the usual suspect, and it is an inexpensive part to replace.
A laptop. Plug in the charger and look for the charging light. No light means the charger, the cable or the charging socket. Try another charger of the right type if you can borrow one. If the light comes on, hold the power button down for a full twenty seconds to force a reset, then press it normally. A laptop that has been flat for weeks may need thirty minutes on the charger before it will respond at all.
A laptop with a removable battery can be tested by removing the battery and running from the charger alone.
Fans and lights, but a black screen: the display
Is the screen on? Monitors have their own power and their own input selection. Check the monitor's light, press its input button to cycle through HDMI and DisplayPort, and check the cable at both ends. Try a different cable, or a different monitor or TV, to find out whether the computer or the screen is at fault.
A laptop with a black screen may simply have the brightness at zero, or may be sending its display to a monitor that is not there. Press the brightness key, and press the key that switches displays, usually a function key with a screen symbol. Shine a torch at the screen at an angle; if you can faintly see the desktop, the backlight has failed and the screen needs repair, but the computer is fine and everything on it is safe.
Try a hard reset. Turn it off, unplug everything except the power, hold the power button for twenty seconds, and start it again.
It starts and then stops: start-up
Restarting in a loop or a blue screen usually means a failed update or a driver problem, not dead hardware. Windows will offer Automatic Repair after two or three failed starts; let it try, then choose Startup Repair, and if that does not help, Restore from a restore point.
"No boot device" or "operating system not found" means the computer cannot see its drive. Sometimes a USB stick left plugged in is the cause; remove everything and try again. If not, the drive may have failed. Stop trying to start it if the files on it matter, because repeated attempts can make recovery harder.
Beeping or a pattern of flashing lights with nothing on screen is the computer reporting a hardware fault in code. Memory that has come loose is the commonest cause on a desktop; on a laptop, the code is worth looking up for the model.
A "recovery" screen on a Mac, or a folder with a question mark, means it cannot find its startup disk. The Mac's built-in recovery, started by holding the power button, can repair the disk or reinstall macOS without losing files, in most cases.
What is safe to try, and what is not
Safe: everything above. Reseating cables, trying another charger, monitor or cable, holding the power button, using the built-in repair tools.
Not safe if the files matter: reinstalling Windows or macOS from scratch, running "repair" tools that rewrite the drive, or opening the case if you are not sure what you are doing. If the drive is failing and the files are not backed up, the priority is getting the files off it, and that is a job to bring to us before anything else is tried.
What it usually costs
A cable or charger costs little. A desktop power supply is a modest part and an hour of work. A laptop charging socket, screen or backlight is a repair whose value depends on the laptop; our guide to what a laptop repair should cost gives ranges, and repair or replace helps decide. A failed drive is replaced with an SSD, and the question becomes whether the files can be recovered, which depends on how it failed.
Afterwards
Whatever the cause, a computer that has just been through this should be backed up as soon as it is running. Our simple backup plan takes twenty minutes and means the next time is an inconvenience rather than a loss.