Question Replaced GPU, desktop immediately turns off

[Tcman2000]

Hi,
I replaced my GPU and now my desktop won't boot. The power button light will flash and the fans will start but both immediately turn off.
These are my specs with the exception of the GPU being replaced with a GTX 1660 Super: https://pcpartpicker.com/list/sjXzTW

My process when replacing the GPU was:
removing the CPU cooler.
Removing the old GPU
Placing in the new GPU
Applying new thermal paste on the CPU
and screwing the CPU Cooler back into place. I did not touch any other component of my PC. I've tried placing my old GPU back and the same issue persists. Any ideas on what to troubleshoot?
I've also tried resetting CMOS if that helps troubleshoot the problem.

Edit: Is it possible for a loose CPU cooler to cause the PC to automatically shut down?

Edit2: Nevermind, I'm an idiot. The CPU power cable was loose and got yanked out without me noticing.

 

[computerjoe314]

If you put the other GPU back in does it go back to working?

 

[Tcman2000]

If you put the other GPU back in does it go back to working?

No, my desktop doesn't turn on when I put the old one back either. Is it possible I messed up my PSU? I flipped the power switch on the PSU but I didn't unplug it when I was replacing the GPU.