Several things, from simply your power cord is loose or the wall jack is bad to bad power supply or bad motherbord. Your system is from about the era where there some issues with motherboards, check the motherboard for bad capacitors, they should be flat and silver. If black dots on them or they are puffy, need a new motherboard. If the motherboard looks OK check the power supply, need to swap for a good one. Since you have 0 activity when you try to turn it on, it's probably the power supply. Another thing, maybe the power button is a bit loose, take off the front pannel and try the power switch directly.