The next step is to remove the two power on wires that go to the motherboard header. Then touch the two pins with a metal object like a small screwdriver for a moment. If it starts up then the problem is the case switch or wires to the switch that sometimes get pinched and broken.
If that fails to start it up, then you need to troubleshoot all the other components.
I wont be able to try this until later today but I will report on if it works. Thank!