This little trick recently saved me from a critical situation and shows one more time the weakness of every system with unencrypted disks.
Press e at grub loading :
Add a init=/bin/bash after the line /boot/vmlinuz (…)
Press CTRL+X to boot. If your partition is encrypted, you’ll be prompted to enter your password. If not, you’ll be directly dropped in a shell with root privileges.
You still need to mount the partition to be able to modify /etc/shadow file.
mount -o remount -o /
Change your password with the basic command :
Just reboot, et “voilà” !