General system instability, but can be caused by OC of your system/CPU/RAM, graphics card, heat issues that contribute to both. It can be a host of other things, but since you were playing a game that tells me its a sympton of one of the above. Check your system and heat temps, or better yet use a thermal monitoring program that allows you to record temps. Then run the game and exit out after a bit. Check to see what temp ranges for your CPU and ambient case temps. If they seem normal, you're probably going to have to back down on your OC's til you see what the problem is.
Chiz