Black Screen Critial Error

Hey, sometimes when i tab out of the game and then tab back in i get a critical error of some kind, that i can't read because it locks me into a black screen that can't be changed. I get a glimpse of the task manager when i try and tab away and it is league of legends that has crashed. I don't know why this happens as I've never had a problem alt-tabbing before, y'know when I'm dead or whatever but it's put me in leaver buster cause it happened during the load into the game and counted as an abandon cause I couldn't recconect in time from restarting my PC and it happened again, loosing me a ranked game. Any and all help would be appreciated