LoL crashes when I launch

Every time I try to launch LoL , the game crashes. It does not a thing. If i try and open LoL it tells me another instance of that program is running, and displays a black square on my screen. I can shut LoL off in my task manager but trying to restart it inly causes the same series of events to happen again. I tried to uninstall/reinstall LoL but alas, still no fix for my problem. This has just started happening since I downloaded the new beta release. Help please!