En cas de perte de mot de passe :

If you want to set no password, don’t remove the hash in /etc/shadow. Instead go into /etc/passwd and remove the “x” from the line which starts with root, like:
root:x:0:0:root:/root:/bin/bash → root::0:0:root:/root:/bin/bash

After rebooting and log in, just type sudo passwd root to set new root password.