Optiplex GX620 Flashing Yellow Power Light and a Dead Computer

I have a GX620 purchased in April '06 that seems to have died. On the front panel, the power light flashes yellow and the computer doesn't power up when the power button is pressed. The computer has power going to it. All the connections from the power supply to the motherboard seem snugly connected and correct. The amber light on the motherboard is lit and steady and the ethernet connection light has one light steady orange and the other light flashing yellow. Nothing is lit next to the front panel headphone jack. The computer was generally left on all the time, but was connected to a good quality surge suppressor. The network connection was surge suppressed at the corporate network closet.

 

Any ideas on how to determine what happened and how to fix this?