Why is my computer randomly restarting?

Check that the power cable is securely connected to the power outlet and the computer.
Check that the computer is not overheating by feeling if the fan is running and the sides of the computer are warm.
Check for software issues by running virus scans, updating operating system and drivers, and making sure latest security measures are in place.
Check the hard drive to make sure its properly connected and not damaged.
Review power settings to make sure computer is set to stay on when there is a power interruption.