Happened to me only 3 days ago, it was working perfectly the night before but in the morning, nothing.
Took a chance and bought a new case with a psu as i suspected the psu and the price was the same for just a psu...and it worked...phew.
I'd been meaning to buy a new case anyways as the old one made the psu sit around about a centimeter above the cpu fan, the temperature difference now is amazing.
edit..plus it looks nice and flashy now with purple holographic powerlights ect and a see through side... for £35.00 not bad in rip off UK.
I looked up "gormless" in the dictionary, it said "without gorm" .... so i looked up "gorm" but it wasn't there ????