so this kid ive built a computer for is on his 3rd mobo.. originally we tried a new psu when the first mobo wouldnt power up, and the second mobo fried, so for this 3rd replacement i put the original psu on and flipped the switch. green light is on on the mobo, but when i pressed power, nothing happened. so i played with it a few more times and got it to start. turned it off to try again, and now it wont start. i took apart the power button assembly incase the external button wasn't making contact with the actual switch, but its still not powering up.

any ideas?