Quick and easy: Reset root password

  1. Reboot machine and press <shift> to configure GRUB
  2. Select the entry you normaly boot, e.g. Ubuntu and press <e> to temporary modify the entry
  3. Search the line which begins with linux /boot/vmlinuz[...]
  4. Remove everything and including the ro, e.g. ro quiet splashor ro maybe-ubiquity
  5. Replace it with rw init=/bin/bash
  6. Press F10 to boot the system
  7. Use passwdto change the root password