Optiplex 7080 Tower no power.

Remove the CMOS battery and leave it out of the system for 15 minutes. Then reinsert it and try to boot. If you are not able to boot then go to the next step. Press the test button on the PSU. The green LED should light up and the fans should start. If that does not happen then the PSU more than likely needs replacement. Now if the light and fans do come on then disconnect everything from the PSU except the MB and try to start. If the system boots then reconnect the drives one by one and try to start after each one is connected. If it works until one of the drives is connected then you found the problem, a bad drive. If that does not work then remove all of the RAM and try to boot. You should hear beeps or get diagnostic light error codes. If you do not then the MB may need replacement.

This support page should help with PSU diagnostics.

Perform a hard reset
Inspect the power cable and the power port on the desktop for damages
Verify that the power cable and wall outlet are working correctly
Check LED indicator on AC adapter (select Dell all-in-one computers only)
Verify that the correct voltage is set on the power supply unit (applicable for Dell desktops only)
Run a diagnostic test on the power supply unit (PSU) (applicable for Dell desktops only)

Regards,
john

All previously done, still not powering up. Diagnostic test on the PS passed. CMOS battery was removed for about 24 hours to clear and still not powering up. Only thing left on the board is the M.2 drive and the PS.

may need to perform a bios recovery, but if pc is within warranty, send back to Dell repair depot for warranty repair shipping cost included.

How do you do a bios recovery if it's not powering up? I have the bios recovery on a USB stick but again, not powering up.

May need to pursue as warranty issue/repair/return/exchange.

Good point.  Re: hit the power button it came on for 2 - 3 seconds then powered off and no response from the power button.

does sound like a motherboard problem.  mobo powers on x 2sec then turns itself off is a sign of faulty board.  Therefore back to warranty repair.