If your iMac is the G5 1.9GHz MA063LL/A (Machine ID: PowerMac12,1) you may have the bad capacitors issue that caused power supply and logic board failures. If you can't recover from usual troubleshooting methods then open the case and check the logic board capacitors. Here's documentation about the iMac G5 capacitors issue:

http://en.wikipedia.org/wiki/Capacitor_p...

http://www.flickr.com/photos/30825931@N0...

http://lh3.ggpht.com/_hsvnxFEc2do/SzxOvT...

Try plugging into an external display to see if the picture appears.

Do the fans spin at top speed until you turn it off? And, once it's on, can you turn it off with the power button? If they do, and you can't, it's most likely a logic board problem

I have a white iMac "Core 2 Duo" 2.16 20-Inch (Late 2006) with the same problem. I started with the screen going black but the computer still on but after a while everything went off completly as if i unplugged from the power source.

I haven't taken to the Service yet, but i talked to them and they said if it the Internal Display Power Supply or the Inverter it is a very expensive repair. They probably DO charge high anyhow, not many good services in Spain.

So does anybody know a trustable source of getting spare parts of iMac in the States? I did check eBay but could not find a bit on iMac Intel White, there is more stuff on G5 models.

Cheers,