Black screen on iMac 2374

Hello, My Imac EMC 2374 Late 2009 27 inch Imac Screen went black and I need help diagnosing it.

Information:

I have restarted numerous times (Including disconnecting the power cord and Pram starting).

I get a chime at start up. The White screen and Apple logo appear and the line that shows it's loading starts to fill. Then the logo and the line disappear with the screen remaining white for about 10 seconds. Then everything goes black.

I connected an external monitor. This time the screen remained on until my desk top with icons finally appeared on the imac (the external monitor, an HDMI TV, did light up blue (the color of my actual desk top) but the icons didn't appear on that screen) Then everything went black.

When I tried that again I couldn't get the Imac to start at all and only 1 led remained on.

I waited about 10 minutes to retry and this time it restarted with the desktop with icons appearing. Again after 10 seconds it went black.

Any help or insight would be greatly appreciated.