Processor hit it's max thermal limit and shut itself down to prevent the cores from burning. If your running an Intel chip download this -
http://www.techpowerup.com/downloads/1691/mirrors.php if it's AMD use this one instead -
http://www.alcpu.com/CoreTemp/Core-Temp-setup.exe
Run those while running prime95 and watch your temperatures climb. If they go above 80 degrees you have to either get a better heatsink/air cooler or clean your old one and apply some new thermal compound.
It's a simple process really, shouldn't take more than an hour