Hmm, you could try pushing the "reset" button instead, just to see if that works (Ive heard countless times of people accidentally getting those to mixed up, including myself )
If that doesnt work, check to make sure all wires are in the correct spot and pluged in securly. If that doesnt work, your PSU might be a lemon
is this you're first attempt to powerup, i.e. has it worked before? if it is the first time, make sure the psu switch is on, all power cords plugged in, and if the power and reset buttons are hooked up.