Could be a blown cap, regulator, south bridge, fuse, resistor, a million things. Hard to diagnose on a board and not something I usually do unless the owner understands I will charge whether I can fix it or not as it can be hours with a multi-meter trying to track it down and then it may turn out to be a part that you can't replace anyways.
Myself, I would start measuring with the power plugged in around the power input jack and see if it's getting power and then trace that down to see if the main parts seem to be getting the main power from the power jack. If that's all good, which it probably is in your case since power wasn't even plugged in, then it's hit and miss to figure out.