Click the start button, type in "view event logs" in the search box, and click "View Event Logs in Control Panel".
Click on the Windows Logs - and Click System. Scroll through looking for the date/time it shut down, there should be entries as to why it shut down. If there aren't any entries - Windows didn't shut the computer down.
When windows doesn't shut down properly - it would have asked you if you want to enter safe mode and/or do a Windows recovery. If this is happening, you more than likely have a bad PSU (although it could be other components).