Client crash and computer shutdown

I'm not exactly sure when this first happened. As soon as the loading screen is done and the game (summoner's rift) loads, after about a second, my screen goes black, a loud buzzing comes from my headset and there is a loud fan whirring sound coming from the computer. There has been some times when the game has loaded, and any amount of time into the game this crash will happen. I usually can't rejoin when my computer turns back on because I get the same crash.

I've tested in a game with bots and it didn't crash, so I tried a ranked game again but got a crash. I've since tested with bots again and got the crash. I've received a warning/penalty for leave games because of these crashes as well.

I'm asking about this here, because League of Legends is the only thing on my computer that has caused this crash.

I have reset my graphics card software, and repaired corrupt game files.

Any suggestions for a solution?

My graphics card is a NVIDIA GeForce GTX 560. Processor: Intel(R) Core(TM) i5-2500 CPU @ 3.30GHz RAM: 4.00GB

DirectX 11.0