Power switch button

[Trotter]

Try beating on your brother until he confesses what he did to it.

Could be the mobo, could be a component not plugged in right/completely, could be the power supply. The switch sounds like it is working and hoked up right.

Does the mobo have a BIOS speaker on it? In other words, did it used to beep when you booted up the computer?

 

[kmote]

All motherboards I have seen have an LED on them to let you know when they are receiving power - no light, no power. I would bet on the PSU.