iMac Black Screen On Bootup

I started up my iMac today and heard the chime but all I get is a black screen. I've tried unplugging all cables, Resetting PRAM (Which did not work, only get one chime) Any suggestions? Here is a video of what happens when I start it up

http://s4.photobucket.com/user/drumkey87/media/macnoboot_zps7eddaae7.mp4.html

Do you have a spare monitor or TV you can hook up to your iMac? That would tell us if there's a problem with the iMac display, backlight, or graphics card. Try using a flashlight and shine it directly on the iMac display to determine if there is any video present.

The good news is that now I got the display to turn on but after a while it turns off and then I cannot get it to come back on anymore

I would take this to an Apple Store or Apple Authorized Service Provider.