*This is based on my opinions and experience, don't flame me, it worked for me.
Before you start, is your console modded? That is the problem.
1.Power Supply!
1.Unplug and replug, did it help? If not continue.
2.Try another socket/room. If that fixed it, stop here!
3.Disconnect and reconnect ALL of the wires. If it is fixed stop now, and go on with life.
2.Overheating.
1.Did you do ANY of the following;
1a.Mod the console.
1b.Play for more than 8 hours (at a time.)
1c.Hit the console (may caused something to break and short circuit the poor bugger.)
1d.Leave running for more than 8 hours.
So, did you?
If so, you can; Void your warranty and "do-it-yourself" style fixing. OR, Send it to microsoft.
3.Piracy
Did you pirate a game? If so, then the XBox disables itself. (Happened before, trust me.)
4.If all else fails...
Ragequit and beat the stuff out of the thing! Send it to microsoft for repairs!
Sorry for posting a little to much today.
Edit:Good call, changed name.