Cloudy_KiTeer

Is your old PSU one with modular cables, and you just plugged its modular cables into the new PSU? If you did, well, that's what caused the damage. NEVER EVER PLUG ONE MODULAR PSU'S CABLES INTO ANOTHER ONE. They are not compatible. Always use the cables that came with the PSU.

Only by unplugging everything and trying one thing at a time. Disconnect all fans, GPU, drives, if you have multiple sticks of RAM, remove all but one and see what happens.

I wouldn't be surprised if it was everything.
 
If he's lucky it's just the main board, but.... well it's really dependent upon what was wrong.
 
Pro-tip, you can actually use your nose in situations like this. Sniff around for burnt plastics on the various components. I've had to do that to diagnose system boards in laptops, in the past.