Computer has power for a quick second

AMD FX 8320 8 Core 3.5 GHz (Black Edition)
ASUS M5A99FX Pro R2.0 Motherboard
8GB DDR3 1333MHz Corsair XMS3
Radeon HD 6950 2GB
120GB SSD & 512GB SSD

Okay, so my desktop randomly stopped working.

The power button does nothing. When I plug in the power cord while the psu is switched to on, the fans spin and lights light up, but only for a quick second.

I've tried swapping PSUs with no luck.

Please help, thanks!