Boot to the GRUB menu. Then, press (for edit) before Linux has a chance to boot.
Using your keyboard arrow keys, move the cursor down a line or so, and press (for edit) again, you should now be on the kernel line, and press to go out to the very end of that line. Now hit the spacebar once, if necessary, to add a space, and then add one of the folllowing:
init=/bin/bash #or init=/bin/sh
Then press , and you’re back at the kernel line, press “b” (for boot) and the computer will proceed to boot to a root prompt.
At this point, the root file system is still mounted read-only, so enter the following command:
mount -n -o remount,rw /
Enter your new password. Retype if asked to confirm.
Then restart the machine by typing:
Now you should be able to login with your new root password as required.
Short link: http://taxze.com/?p=90
Canonical link: http://www.taxze.com/recovery-root-password-debian/