So, weird thing happened...

I was casually browsing the interwebs, when all of a sudden my display goes black, and my computer powers down...When I turn the PC back on, all of the programs I had open (chrome, WMP, Notepad, Windows Messenger) were already open, it didn't do a normal start up or anything. I'm completely confused, I've never even heard of this happening.

i5-2500k
GTX 480
Corsair GS-800
ASUS P8Z68V-LX
WD Caviar Green 1TB HDD