I have a 2009 core i7 27" iMac running snow leopard 10.6.8. This morning it won't boot up. No matter what I do, I can't get past the apple logo screen. I've tried booting in safe mode, running hardware test, booting from the install cd, and nothing, never gets past the apple logo screen. Sometimes I will get the little spinning cog under the logo but the cog always freezes, most of the time I don't even get that, just the apple logo. Does anyone have any suggestions, or thoughts what it could be? All help is appreciated.