That's an extremely vague description of the problem... When does it restart (as in, what are you doing on it when it restarts, usually?) How old is it? What brand/model? Do you have antivirus software on it? Please give us more detail please
That is what we lovingly refer to as a "crash". Could be any number of things, from memory that needs to be reseated, to overheating of some component, to a software or driver conflict, or a bad PSU supplying a weak or "dirty" power flow.