that means your system had a "critical" error and your PC is set to automatically reboot when it comes by a critical error..

If you want to find out what the error was about next time, then do this : right click on my computer > properties > Advanced tab > under startup & recovery, select "settings" and uncheck the box next to "automatically restart" .. next time an error occurs, it will show you the BSOD instead of just rebooting..

but if you are still wondering what it was about, then you can check the Event viewer, a service on windows that logs every application, system errors etc..

To reach this, go to your control panel> administrative tools > and open event viewer.. look under the "System" and it should show an error marked with a Red X , and that is most probably why your computer rebooted by itself..

I hope that helps

Blackwar