Logically we must eliminate the various possibilities.
New rig....can be a likely card seating.
Pull all cards out...both video (I assume you did a complete assembly).
I know...no card, no visual.......not a problem yet.
Unplug all hard drives, etc....everything.
Now...you're going to think I'm crazy....but this has happened.
If you have a PS/2 keyboard.....pull it out!
I've seen them do this before!
Ok, nothing but motherboard......power on.
If the fans turn off now, you're going to have to dig deeper.
If that turns on and stays on....you'll hear beeps regarding no video I expect (no IGP I assume).
If it did turn on, the one of the other devices is a problem, likely the mounting of a card (new setups can do that)....I've had to include spacers to raise the motherboard a mm or two to make things fit for real, even on good cases.
If that DIDN'T work, you'll need to pull RAM chips and try that with none...none!
It will beep and complain, but the fans should run - or at least the behavior should change.
One RAM chip...try again.
Then all four....
After that....you've got to either test the power supply in another board to be sure of it (odd since you've switched).....
Then...yep...pull the board and try it outside the case.
One other thought.....are you using a UPS?
Try powering without it once.