1. What pc do you have?
2. Operating System?
3. Any pattern to shutdown? (after set period of time etc)
4. Have you checked your Power Saving settings? (including hibernate, sleep etc)
5. Checked for over-heating? (fans working, inside of pc clean of dust and fluff)
6. All cables firmly in their sockets?
7. any error messages shown?
8. does it just restart back to the same screen that you were on before or is as if you had just booted up? (Windows loading up (to login screen if you have it set) from scratch before desktop shows)