kernel init

• Boot your system to the grub menu. ESC
• Edit the menu entry with E
• Add at the line starting with linux this:

  init=/bin/bash

• Type the function key to boot.
• Now at the bash session remount the root filesystem writeable:

  mount -n -o remount,rw /

  Now you have root .... careful now

Surely dangerous, remember though - you must have physical access to the machine to do things like this.