|It sounds like it could be a power supply but really, it's hard to say without digging a little deeper.|
I remember having a similar problem with my old PC where it would stop and start randomly. That was down to the power supply. Are you able to load up the operating system at all, or does it always restart and lock you out?
Have you tried installing any new hardware recently? That can be a tell tale sign if the problem is sudden and unexpected.
The Definitive Guide to Registry Cleaners &