I dont have one i just got it 30 mins ago I even opened up the unit I saw nothing wrong with it from what I could tell?Have you tried it with a different PSU yet?
One of the first things to try would be to swap the PSU for a known working one just to rule out the PSU.
I wouldn't open up the PSU if you don't know what you're doing.
It carries high voltages through its components which can be lethal.
If you do fancy on making a dirty repair, I know there's a ATX PSU to Xbox PSU mod.
Anyway, I doubt unplugging the PSU would work since you said you just bought it.