Won't boot normally or in recovery and single user mode, safe mode hangs up halfway
iMac locked up with visual glitches while playing the Mac beta client of League of Legends. Had to force a shutdown by holding the power button. Now it won't boot; after the Apple logo and spinning wheel I just get a white screen. I tried launching in recovery mode, that didn't work. Then I tried safe mode; the bar filled a little over half and then nothing but white screen. I was able to boot single user mode once; ran fsck -fy and rebooted, still white screen. Now I can't get back to single user mode. I booted with command option P R to reset the NVRAM and got the second chime, but still nothing.
Luckily all my important files are in the cloud, but I'm really hoping my computer isn't completely dead. I haven't had any issues in the past. I've even been playing LoL for over a month with no issues, until now when I tried to play the newer game types, Dominion and ARAM; both caused visual glitches forcing shutdowns. I was able to restart my computer with no problem after the first two visual glitches, but after the last one I am stuck at the white screen as I described above.
Thanks in advance for your help!
iMac (27-inch Mid 2011), OS X Mountain Lion (10.8.5)