I had exact same problem.So i had a poke about on the net and was checking the capacitors as to which ones to replace.
I decided to pull the connector from the psu to the board out and re connected after around 15 seconds and bosh!my baby fired up again.
I quickly re flashed with openatv 6.3 and all is still good 3 days on.
So check the psu connector and hard drive connectors to.