Well, since you say that it does indeed boot up properly then there are 3 likely options: damage to the case power button/wire, damage to the motherboard pin(s) where those connections go in, or (most likely) you didnt plug it in correctly. If it;s the first one I think you could probably fix it but it's more effort than I would go through/ more expertise in that area than I have, if it's the second one then you might need to rma/return the board if you don't want to jump the connection.
However since the board apparently works, I would just check, and then check again that the right wires are plugged in to the correct pins