Compaq desktop pentium 4 intel processor inside doen’t start?

i have a compaq desktop pentium 4, running at 1.5ghz, it doesn;t start yet its powersupply and the switch are fine. please help me troubleshoot it and fix it.
however when i try switching it on the lights next to the switch remain green with no display on the monitor or lcd desktop and put it off i have to remove the power cable.
i will be grateful for the assistance offered.Dpg213