did you open up your xbox? if so check to see if the wires from the power & eject are attatched to the mother board. it also could be your powersupply or the power & eject ic board if u need parts check on ebay thats about all i kno to my knowledge, let me kno if it works out! mike
If it's none of those mikenh listed and your xbox is a version 1.6, a common problem I've seen could be the transistor on the motherboard.