Mac pro crash! w8ing online

I have this problem today, I was playing LOL with my Mac pro 13' (using mac system), then suddenly the screen turned to black, and my laptop crashed. I have to press the button to restart, but after I restarted, I still can't play (but can log in), everytime I tried to play the screen turned to black and the laptop crashed, I've also re-download league of legends, can someone tell me why and how to fix it.. w8ing online!!!!!!!!!