Need to access F12 boot menu on Optiplex 3080 again

One time boot menu does not mean it only works once. It means that you can change the boot menu one time instead of the auto boot menu set in the BIOS. That one time change can be done at any boot though. The option you select does not modify your predefined boot order settings. I assume you tried F12 and it does not work. You can press F2 at boot to change and save the boot menu until you decide to change it again.

This web page may explain it better than I have tried.

Nothing.  Everything we do, F2 and F12, do not work.  It shows the Dell logo then immediately starts to boot Windows.

I take it you press F2 or F12 immediately upon startup (no waiting for keyboard light), and repeatedly?  

Might be time to reset CMOS which also resets BIOS.  If the battery isn't more than 5 years old, remove it for 10 min., reinstall.  If it's more than 5 years old, replace with new.  A bad battery can cause all sorts of seemingly unrelated bizarre problems.

Try power off, disconnect power cord, drain power by pressing power button 10 sec, remove CMOs battery from motherboard, let system sit without battery for 30 min, reinstall CMOs battery, test F12 again.