HP Pavalion will not power on

Hello,
I have an HP Pavalion desktop running Vista. When I went to use it today it would not turn on. I never turn it off. So. I was surprised when it was not on. I read on the Internet to Hold the power button down until it turns off and the unplug it for 5 min and plug it back in. That did not work. When I pressed the I/O Power button on top it glowed light blue for a second and then did nothing. I think the power cord is OK because the little green light is glowing on the back of the tower right under where the power cord plugs in. I also took it apart and cleaned the inside so there was no dust. Anyone have any idea?

Thanks for your help