why a computer restarts abruptly?

Clarifications

Check the system board for bulging capasitors.
If you see any, that is your issue.
Not sure what time of computer you have but this is a known problem with certain models of Dell's and IBM's.

Short in the electrical cord
Old Fuses
Bad Fuses
No Fuses
No chip fields to support the stuff you are using
Not enough memory Volumne {Cubic Inches for instance}

Horrors!!!! !! Someone else with more authority is useing your above stuff more importantly than you, how do you know? all the opter stuff aint a problem.

Enta Wanabi

On some systems, if the CPU is overheating due to dust on the fan/cooling tower or the fan itself has stopped, the system will reboot when it gets too hot.

On other systems, it just plain shuts down and won't restart until it cools off. Then it goes through the same cycle.