Dimension 8400 no power

I have a Dell Dimension 8400 Home Media Edition Windows XP SP3.

Just recently I turned my computer off to give it a day of rest and had trouble getting it to turn back on using the power button. I had to keep trying to press the power button and it finally booted up. However, I turnjed it off again and now it will not power up. I had absolutely no problems prior to this.

I have replaced the CR2032 battery even though I had no trouble with configuration and date/time. I am thinking the problem is either the PSU or Power On switch. I do not know how to remove the front facade to get to the Power switch nor do I know how to test the PSU

I would appreciate some steps to take in troubleshooting this. Here are my symptoms:

I have plugged the power cord into a known good outlet to remove the power switches or UPS as being a culprit. No Change.
I have Reset several boards such as video and TV tuner (these were in my way to get the the CR2032 battery). No change.
There is a little green LED on the mother board that comes on when power is plugged in, goes out when unplugged.
There are no Diagnostic LED lights at the rear of the unit. They do not flash or give any indication of ever lighting up.
The power on button does not light up at all.

What is the best way to proceed?

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