SOURCE: akeness.blogspot.com

You can log in using single-user mode and create a new root password.

To enter single-user mode, reboot your computer. If you use the default boot loader, GRUB, you can enter single user mode by performing the following:

   1. At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.

   2. You are presented with a prompt that looks similar to the following:

      grub append> ro root=LABEL=/

   3. Press the Spacebar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:

      ro root=LABEL=/ single

   4. Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:

      sh-2.05b#

   5. You can now change the root password by typing

      passwd root

Here is my step by step screenshot resetting my own root password.

step 1
step 2
step 3
step 4
step 5
step 6

Enjoy!