Sounds like something's blown and gone short-circuit (or close to that state). The net result is that as soon as you've switched the PSU on the resulting current draw is way too high and trips the PSU over-current protection circuit and switches it back off. It's probably something on the motherboard, but just to make sure you could unplug all the expansion cards and disconnect all the drives (and their power connectors), and even take the RAM off the motherboard, so that all you're left with is the PSU connected to the motherboard. If the PSU still won't stay on, it's the motherboard!