Originally Posted by Zealotry

If you have console, connect and do this:
Reboot and enter GRUB
Select the kernel
Press the e key to edit the entry
Select second line (the line starting with the word kernel)
Press the e key to edit kernel entry so that you can append single user mode
Append the letter S (or word Single) to the end of the (kernel) line
Press ENTER key
Now press the b key to boot the Linux kernel into single user mode
At prompt type passwd command to reset password:

You need to mount at least / and other partitions:
# mount -t proc proc /proc
# mount -o remount,rw /

Change the root password, enter:
# passwd
Finally reboot system:
# sync
# sudo reboot now