Why did you my computer just restart itself?

Question

I was watching a clip in Windows Media Player. Suddenly while its playing, my computer restarts itself. There were no messages before it shut itself and restarted, and there were no messages when it booted back up. Suddnely the monitor goes black, and the computer just restarted.

I doubt it was overhearting and I have no viruses, would anyone know why this happened?

Answer 1

Or your PSU is dying on you.

Answer 2

overheat.  

Answer 3

Could have been an electrical problem like a short brown out.