Record the circumstances
Note the time, running applications and whether an error screen appeared. Distinguish a scheduled update message from a sudden loss of power. Check whether restarts happen during games, at idle or after connecting an accessory. These observations help guide diagnosis but cannot by themselves establish that the power supply or motherboard has failed.
Protect documents first
If the computer still works reliably for a while, copy important files to separate storage. Avoid repeated stress testing when it keeps shutting down. Photograph error codes and list recent changes. If a drive disappears, makes unusual noises or produces copying errors, stop testing and discuss data safety before proceeding.
Software versus hardware
A fault that begins after a driver installation may be software related, but timing alone is not proof. System logs help establish when operation stopped; a general unexpected shutdown event does not identify a specific failed component. Review drivers, updates and the previous stable configuration before assuming that a complete operating system reinstall is necessary.
Cooling, memory and power
Blocked ventilation, unstable RAM and power problems can produce similar symptoms. Keep vents clear and avoid operating a laptop on soft surfaces. Do not open a desktop power supply. If restarts began after an upgrade, list the changed components and settings. There is no single temperature value that diagnoses every computer model.
Targeted service testing
Testing follows the symptoms and can cover power delivery, temperatures, memory and software. Reproducing the conditions is more useful than replacing parts at random. Agree the scope of any work affecting files before it begins. Finding an intermittent fault can take longer than replacing a known faulty part, so accurate notes are valuable.
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