Hi,
In this case we need to trial and error method. If you are comfortable to open the system box please go ahead and perform the below.
Open the chassis and remove all the following components, all add-on cards, memory , drives like floppy drive CD drive and hard drive. Power it on and check does system power on.
If the system does not power one it is the issue with the motherboard or the power supply. If the mother board has the power led glowing, then mother board is bad, I f mother board led does not glow, then power supply is bad.
If the system powers on in this bare minimal configuration. (As you could see the BIOS listing at the startup) then add one by of the parts and find the culprit device.
Please remember to wear antistatic wrist wrap while doing so.
All the best
Regards.