Had a
similar issue recently, though I did get power just no post. I bought a
power supply tester, and all checked out fine. I eventually just bought all new parts, psu, mobo, ram, cpu. I decided that buying new parts was better than spending money to replace the old mobo, if I could even find it.
I would start with your PSU, might be able to just take the PSU alone into a shop to have it tested for a couple bucks.