IIRC there is no root password.
Single user mode won't help as it requires root password too, and you need to be root to edit /boot/cmdline.txt in order to remove this.
So you can :
use a windows computer to add init=/bin/sh at the end of cmdline.txt in the /boot (fat) partition of the SD card
use a linux computer (live distro if you don't have one), then mount root partition of the SD card and change ownership of sudoers.