Dell Dimension power problem

Early this morning there was a power outage in my neighborhood.  I have my Dimension 510 on a UPS.  Since the outage didn't end quickly, I shut down the 510 (Win 7 Pro) and everything was fine.  When the power came back on, I turned all my devices back on.  However, the 510 wouldn't boot.  Nothing happens when I press the power button.  I checked the USB ports and there's no power in them.  The DVD drawer won't open.

However, the green light at the back on the computer is lit.  It remains lit even when I pull the power cord out of the computer.  I thought I remembered in the past that this light would go out when I pulled the power cord.

Any suggestions?  I assume the problem is either the power switch or the PSU but maybe there are other possibilities.  If I need to replace the PSU, what size would be recommended?  I have a low voltage graphics card that just draws power from the slot, so I think my power needs are minimal and I haven't had a problem before with the stock PSU.

Thanks for any advice.