Try disconnecting everything except for the MB & of course the power button to the MB. If it works, then you know the problem isn't your PSU. Now, connect something else to your PSU, & try powering up again, & so on until it stops working. If/when it stops working, then the last device you installed is the problem.
I had the same problem last week! Turns out when my last PSU died (there was smoke), it took my Hard-Drive's PCB with it!