Game Crashing PC Mid Game

Alright I need to make this clear at the beginning. I am basically 99% sure that it is actually the game itself that is crashing my PC and nothing else. I could be wrong, but I don't think so.

So I was playing a game as normal today, then all of the sudden about 20 minutes in to the game my screen goes black and my PC completely shuts off. I brush this off as a random crash and turn it back on and am back in the game within 5 minutes. Completely oblivious to me though was the fact that this problem would strike again. About 4 later, and a few games with not interruptions, I made it about 4 minutes into an ARURF game when my screen went black again and my PC shut off. I restarted it again and got back into game, but 20 second later it turned off again. So I yet again turned it back on and got into game, but surprisingly enough about 3 minutes later it shut off again. By the time I turned my PC back on the game was over and the problem had happened 3 times in that one game. Ever since then this problem has not occurred anywhere else what so ever, but only in a league game. The league client itself isn't causing any problems, but it is the game itself that causes it. I have no idea if it is my computer or the game, but I am almost positive it is no outside program other than the game. I am afraid to play anymore games without knowing that this won't happen again, and was wondering if there is any explanation to my problem. Any advice would help and I can provide any further details if necessary.