I'd suspect a bad switch possibly for starters. Try swapping out your power and reset switches and see if you can fire the system using that switch, or a power switch from another case. it could also be the power switch connector worked its way off the motherbord connector.
It could also be the motherboard grounding out to the case, though if it was working before that would be unlikely. in that case i'd look for a lose ***** on the back of the board in the contacts.
you should be at least getting fan power up if the motherboard is still intact and the power switch is working. Even if the chipset on the board is bad i'd at least expect power to the fans if the switch was good.
If none of that works, it could be the board itself, but the fact that it is indicating power would lead me to believe its the switch or a short.