Dimension 9100 won't power up

I turned it off the other day and now it won't do anything. The green light on the motherboard is on. But the power button light never comes on. I jumped the green & black wire on the power supply and the fan does come on. Does that mean the power supply is still ok? Or could it still be the problem? It did this to me several months ago but on a fluke I guess it finally decided it would work because it just happened to power up one day when I pressed the power button.

Thanks,

Darrall