If you somehow loose access to the root user you can quickly reset the password by rebooting to single user mode and running passwd. Here is how.

 

1) Reboot, and when GRUB is presenting the menu list (you might need to press ESC first), follow those instructions:

  • use the arrows to select the boot entry you want to modify.
  • press e to edit the entry
  • use the arrows to go to kernel line
  • press e to edit this entry
  • at the end of the line add the word single
  • press ESC to go back to the parent menu
  • press b to boot this kernel

2) You will now have root access. Type passwd and define a new root password.

 

3) Reboot and you are done.