did you check the old dell psu if it have the same connector dell sometimes use psu made for them and have a way board they use detect only them and work with .
Is it was only PSU upgrade, nothing else, just PSU SWAP?!
I'm sorry for wasting your time! LOL!
I forgot to plug in the CPU power connection. I did that, and she fired up!
Sorry, y'all. Thanks for the help though! (: