I have the same problem .
I was listening music and my Imac should down.
I try to turn it on but the computer should down after the apple symbol. After that no more turn on.
I opened and I notice the fans don't work, no video , no sound.
it have 4 led to indicate if everything is ok. all this led are good. Also I can hear the cd player working when I put some cd inside it.
What should I do?