When you get such a machine you start over. From the beginning.

You pull out the PSU and the motherboard onto cardboard with bare min parts for a power up test. If you can't locate the power on pins of the motherboard in the docs you may be way in over your head but as you know, everything is now suspect.

So back to the beginning with the least number of parts.

If you can't find power on pins, take it to the shop that does your repairs.