My computer frequently stops dead and the screen goes black as if the electric power has been switched off, but it usually then goes through the start up process again as though just switched on. Why does this happen please? Advice appreciated.
Sorry if I'm stating the obvious here but have you checked the power lead is fully pushed into the back of the computer tower? (presuming you are talking about a desktop, not a laptop).
This is symptomatic of overheating. When the behaviour manifests itself, do you detect a different sound from the fan(s)? This could be either the fans going silent, or speeding up dramatically.