The same thing happened to me 3 times today... I'll be playing and all of a sudden my system just reboots. This never happened before Season 4 and doesn't happen in any other game. I'm sorry I can't offer you a solution though.
Edit: I turned off post processing, AA x4, and shadows and was able to play for a few hours without issues so it's probably one of those settings. It stinks because I was able to have those on before Season 4. For reference I have a GTX 460.