My old 2008 iMac won't work. You turn it on normally and you can hear the start up sound and see the Apple logo, but right after a minute or so the screen just goes black, still the computer keeps working, just no image or sign of the OS. I've tried everything to get to recovery mode but nothing works. It just won't load anything, it just go straight to the that black screen. What could it be? Is it really dead or just some OS error?