if you forgot your password root debian server or debian desktop use the following procedure to reset.
- Boot into grub, select single user but do not press enter.
- Press "e" to go into edit mode.
- Scroll down to the kernel line, it starts with "linux /boot/vmlinuz-3.2……."
- Scroll to the end of that line and press space key once and type init=/bin/bash
At this point, the root file system is still mounted read-only, so enter the following command:
#mount -n -o remount,rw /
Then type
#passwd
Enter your new password. Retype if asked to confirm.
Then restart the machine by typing:
#reboot
Now you should be able to login with your new root password as required.
Sumber:
http://www.debianadmin.com/how-to-reset-debian-root-password.html