Google THE DEAD PC and you find the usual tech troubleshooting where you remove parts until it works or it's down to as few parts as you can go.

If you find removing the GPU lets it power up then that's either the GPU or the PSU at fault.
(Note. Acronyms unclear? Please google.)

If you hit rock bottom and there's only a few parts but still dead, then the bad part or parts is what was left.