probably a bad MOSFET transistor (the small black things with 3 legs)
Now you have to determine what maybe caused the fail... so you don't screw up your new replacements.
Possible Causes (in order by chance):
short
static electricity damaged something
too many LEDs drawing current (disallows the power supply to turn on)
faulty power supply, faulty current limiting circuit
protection circuits aren't tripping properly
Does the xbox even turn on?
If the xbox turns on try to see what's the error code. What is it?
If the error code is 0001 or 0002 then one of the MOSFETs are bad.
If the xbox doesn't turn of maybe the RF board isn't plugged in all the way.
If the xbox still doesn't turn on then one of the mail mosfets are bad and it doesn't power on period.
edit: w00t 1000
th post, you're mad.