Diagnose ailing/dead PC

Today I purchased a new gpu and psu. Replaced psu and installed new gpu. No signal and no drive activity light.


Check all cables, remove all unecessary SATA etc, and remove gpu. Test monitor and HDMI with laptop - no issues. Plug HDMI into mobo... wash repeat and cycle through all variables to no effect (reinstall old psu - reinstall old gpu - RAM inspection - pull cpu cooler and inspect entire board for ???) Both PSU power everything up with no outwards signs of problems.


I'm undecided if this was a fun evening or disaster. It was not spent casually gaming. On that I'm quite sure.


Any ideas beyond the mobo or processor are dead? How does one tell which and be sure of it?