Computer will not start

I have several of the same Dell Dimension 5150. One CPU will not start. At first I thought it was the power supply. Replaced the power supply, same result. There are no “beeps” and the power supply fan doesn’t turn on. The mobo light is green. This would lead me to think the power supply and mobo are good? Even with the RAM removed no “beeps”. Could it be the on/off switch or could the mobo be bad despite the green light?