Only those board holes with solder rings top and bottom should be used to mount a motherboard. Avoid use of any other holes, unless use of a fiber insulating washer is added on top and bottom. If a standoff mount cannot be moved, then four layers of electrical tape can cover it. But also use a thin fiber washer to ensure the tape does not get penetrated and cause a short. Also, when installing the board.. ensure those grounding tongues on the back plate adapter are NOT finding their way into LAN or USB ports!!!!!!
If all the physical aspects of the install are proper, then check the 'power good' lead from the PSU. It presents a +5 volt signal to the timer circuits, to allow activation. If too low, no activity. Use only a digital meter to measure the voltage to a black (ground) lead from that gray lead. Recommended only for testing purposes, use a 47-ohm 1/2 watt resistor bridged from a +5 volt source to that lead. If life now appears, the PSU may be faulty.. or there is a problem with the motherboard control circuit. The latter is not easily fixed.
You could do a test setup on a plain wooden or cardboard platform, to see if the board is indeed working. If it worked before, it should still work. But ensure the processor is properly seated, and memory too.. in the first pair of memory slots.
F@H.. to solve mankind's maladies.. in our lifetimes!