Ubuntu, recover root password
You need to load into recovery mode
Then into root shell
Then run passwd root command to change root password.
Check output of passwd command.
If u see something like: Authentication token manipulation error
Try run pwconv command, it should fix this (problem is that passwd and shadow files are not synced)
If pwconv fails with message like: cannot lock /etc/passwd; try again later
There can be two problems, try to find and delete passwd.lock file.
Or in my case, remount file system to have write access with command:
mount -o remount,rw /