BUGG

So i was playing and suddently i went from my 120 fps to 20 fps. i thought it was a lagg or crash i dont know something, but in the end of the game it wasnt. I played 3 more games with 20 fps, then i tried to resolve it. I searched on youtube and here in forum and i couldnt fix it, soo i decided to uninstall. The next day i installed and until now i cant open league, it shows a black screen on client ( where we put our username and password) someone help. :c