Computer Crashes if I ALT + TAB During a League Game

Hello, Recently starting about two weeks ago after a league update or something I would alt tab during the game and immediately my computer would freeze or my main pc background would stop responding and turn completely blue. I love league and I play it all the time but getting shut out of ranked games and put on leaver buster is not my cup of tea. Can someone assist me on something I may do to help this awful problem. If you need any further information let me know. I never encountered this problem before. ~Sean