I had this happen to me. I do not use any cooling devices just the xbox360 as is. It fails to display a picture or play sound. Just before this happened I was playing call of duty 4 and the background faded with a red pigment into the pictures. Rebooting the xbox afterwards resulted in no red rings but no picture or sound on any output from TV/HDTV/HDMI All Failed. So just for you cooling fan types out there it is possibly not related since I do not use one and still managed to get this problem. I have a surge protector on it and I did not have a power flux during this time.

Just FYI