Resetting root password for Ubuntu or Debian takes up to 5 minutes.This can be done by editing the grub options to allow a root shell.

1. Reboot your server choose the regular boot kernel that you use (typically just the default one), and then use the “e” key to choose to edit that boot option.

2. Now just hit the down arrow key over to the “kernel” option, and then use the “e” key to switch to edit mode for the kernel option.

3. You should remove the “ro quiet splash” part with the backspace key, and then add this onto the end:

rw init=/bin/bash

4. Once you hit enter after adjusting the kernel line, you’ll need to use the B key to choose to boot with that option.

5. At this point the system should boot up very quickly to a command prompt.

6. Changing the Actual Password
You can use the following command to reset your password, type your preferred password:

passwd root
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

7. After changing your password, use the following commands to reboot your system. (The sync command makes sure to write out data to the disk before rebooting)

sync
reboot –f

The –f parameter was necessary to get the reboot command to work for some reason. You could always hardware reset instead, but make sure to use the sync command first.

And now you should be able to login without any issues.