LostPassword

From the official Ubuntu LostPassword documentation:

1. Reboot your computer
2. Hold Shift during boot to start GRUB menu
3. Highlight your image and press E to edit
4. Find the line starting with "linux" and append rw init=/bin/bash at the end of that line
5. Press Ctrl+X to boot.
6. Type in passwd username
7. Set your password.
8. Type in reboot. If that doesn't work, hit Ctrl+Alt+Del
9.