Ok, since you left us with almost no info besides the reboot you'll have to be a bit more specific. Please post your computers specifications and the Operating system it is running on.
However, untill that time there are some things you can do. First off, get the latest drivers for your graphics card. Remove your old ones, reboot and install them. Also get the latest version of dx 9 or 10, whatever one you may be using. Then get
Advanced windows care keeper.(always make a backup when the program asks you the first time it starts just to be safe) It's completely free. Install it and clean your pc with it. Also defrag your pc with
Disk defrag, which is free aswell.
All of this may or may not solve your issues, but it will give us over here the knowledge that your pc is pretty much cleaned up and we can proceed digging into your problem from there. Also patch your game to the latest version
here.