PLS HELP BLACK SCREEN POST CHAMP SELECT OSX

iMac on OSX 10.10 Black screen after champ select finishes. Can't exit client and have to manually restart my mac. I've been afk for the last 3-4 games now due to this. pls help i'd like to finish my challenges (also i did try ALT+space, space+enter+delete, Command+tab, all the F4 buttons, basically smashing my keyboard in general too.[zombie-nunu-bummed]