If you forgot your root password for your debian server use the following procedure to reset.

Boot to the GRUB menu. Then, press <e> (for edit) before Linux has a chance to boot.

Using your keyboard arrow keys, move the cursor down a line or so, and press <e> (for edit) again, you should now be on the kernel line, and press <End> 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 <Enter>, 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 /

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.