A computer that restarts on its own is trying to tell you something, and the first job is to make it speak up. By default Windows reboots instantly when it hits a serious error, which hides the very message that would explain why. So step one is turning that off. After that the usual suspects are heat, power, memory, and a bad driver, roughly in that order of how often they are the culprit.
First, stop the automatic reboot
Press the Windows key, type “advanced system settings,” and open it. Under Startup and Recovery click Settings, then untick “Automatically restart.” Now when the machine crashes it will pause on a blue error screen instead of vanishing, and that screen carries a stop code you can search. Half of all diagnosis is just letting the computer finish its sentence.
If the crashes come with that blue screen, the fixes branch off into their own territory, walked through in how to fix the blue screen of death. Come back here if the machine just reboots with no message at all, because a silent restart usually points at hardware.
Rule out heat
A machine that reboots after ten minutes of gaming or video, then behaves while idle, is almost certainly overheating. When a chip crosses its safe temperature the computer cuts power to protect itself, which reads as a sudden restart. Listen for fans screaming, feel whether the case or the underside is hot, and pay attention to whether the crashes track with hard work.
The fix is usually dust. Fans and heatsinks clog with it, the airflow dies, and temperatures climb. Clearing it out solves a great many mystery reboots, and the full cleaning routine lives in fix an overheating laptop, which applies to desktops too.
Suspect the power supply
On a desktop, random reboots under load with no heat problem often mean a dying power supply. As a PSU ages it struggles to deliver clean power when the machine demands more, and the computer drops out. This one is hard to test without a spare unit, but the pattern is telling: fine while browsing, reboots the moment something heavy kicks in.
- Reboots only under load, temperatures normal, points at the power supply.
- Reboots at random even when idle, points more at RAM or a driver.
- Reboots right after you installed new hardware, suspect the new part or that the supply cannot feed it.
Test the memory
Faulty RAM causes exactly the kind of unpredictable reboot that drives people up the wall, because it fails on no schedule. Windows has a built-in checker. Press the Windows key, type “Windows Memory Diagnostic,” run it, and let it restart and scan. It is slow, so leave it running while you get coffee. Errors here mean a memory stick is going, and on a desktop you can often narrow it down by testing one stick at a time.
Undo the last change
If the reboots started right after an update, a new driver, or a fresh piece of software, that is your prime suspect no matter what else the list says. Think back to what changed just before the trouble began. Rolling back a recent graphics driver fixes a lot of these, and Windows keeps a System Restore point you can roll back to if the timing lines up. When a slow, unstable machine is the broader complaint, the groundwork in how to fix a slow computer pairs well with this.