I made a computer for a guy, and when I installed the motherboard, I put in all of the mounts, except for one (the hole for the mount was blocked by an IDE port). He said when he got it, it woudlnt' turn on. Is it possible that by not putting just one mount in, that the board grounded out.
Actually, you may have put in to many mounts and shorted it. Cases usually have more holes for mounts then a motherboard does. ATX boards have 9 places to screw down to a mount, and there should only be a mount behind those 9 holes. If you put mounts in other places you may have shorted something.