I had something like this once, in that case it was front io connected to wrong pins on motherboard.
Buildapc
Reseated them again and made sure they were all facing the same way now, per the book, and still nothin
On the off chance your front io buttons are broken, try bridging the contacts with a screwdriver?
Also, you may want to test your PSU by bridging the green wire to black, on the large 20 pin connector (disconnect first to be safe)
The board has a power button on it as well that also doesn't respond, the RGB and buttons on the board light up though nothin
Pretty unlikely that the board button and the case button are both broken, you may have a brick on your hands.
Worth testing the PSU anyway, in case only one of the voltages is broken. But you'll need a multimeter.
Are you sure you used the right number of standoffs underneigth? No shorts or anything?