iMac 24" Shuts down immediately after chime
iMac 24" White Late 2006. EMC 2111.
When I turn it on you can hear the fans and HDD rev up.
The opening chime plays.
Everything turns off after displaying for a fraction of a second the grey screen with the apple logo.
Power button unresponsive (wont turn back on) till I unplug it.
I'm going to leave it unplugged for the next 2 days and try some trouble shooting steps after.
Does anybody know what might be happening? I have searched and saw posts about not getting to the chime, or hangs after the chime or unexpected shut downs, but nothing specific to what's happening to me.
iMac, Mac OS X (10.7.5)