My PC will not power up. Green light on the back shows power there; pressing start button has no effect.

Carl Anderson, March 2011

I assume that it did start before.

There is a connector that goes from the motherboard to the power switch of the case.
It's possible that that connector got loose.
If that's the case, you only need to open the case and reconnect it.

Check your manuals to find exactly where the connector is located.

Electro, April 2011