Recovering a Linux root (or sudo user) password is pretty easy, but the hard part is getting into the BIOS if you have an Intel NUC PC.
The key combination that you should try is to repeatedly press escape or F12 and hope you see the menu.
Once you have the menu, press e to edit.
Move to the line that ends with ro quiet splash (it’s the long line) and remove the word splash.
In place of that, rather put rw instead of ro and append this: init=/bin/bash
Now press Control+X (or F10) to boot.
You will see a prompt:
:#
Type this:
mount -n -o remount,rw /
now do this:
passwd username
There we go, Bob’s your Uncle.
Reference
https://en.wikibooks.org/wiki/Linux_Guide/Reset_a_forgotten_root_password