Help! My MBP won't boot up - stuck at gray screen!

Well, a few days ago I noticed that sometimes it would get stuck at the white/gray boot screen and wouldn't boot up. So I just shut it down and booted it up again and it would boot fine. But now, no matter how many times I try, it won't boot up! I tried resetting pram etc., repairing permissions, repairing disk, even reinstalling OS X ML using the recovery partition!
This is exactly what happens when I try to boot it up:
Normal apple screen with spinning wheel
Then spinning wheel goes away, just the apple logo
Then the spinning wheel appears again
The spinning wheel goes darker for some reason
The spinning wheel freezes
The spinning wheel looks like a frozen black sunflower of some sort
And it is stuck there

However, my Bootcamp partition still works fine.

What's going on with my MacBook Pro (late 2011, dual 2.8 i7)

Thanks in advance