Power supply on ATX motherboard

I have an IBM compatible with an ATX motherboard. I am loading win98, but sometimes the power just don’t want to come on even though there is a lit green light on the motherboard. Sometimes if I leave the computer alone for a few hours, it might come on when I try. Is there a protective circuit on ATX motherboards to stop u from turning the power on and off? or, Do I have a power supply on the way out? or, Is my motherboard defective?