If you forgot you password for your ubuntu system you can recover using the following steps

Turn your computer on.

Press ESC at the grub prompt.

Press e for edit.

Highlight the line that begins kernel ………, press e

Go to the very end of the line, add rw init=/bin/bash

press enter, then press b to boot your system.

Your system will boot up to a passwordless root shell.

Type in passwd username

Set your password.

Type in reboot


you can find a lot in google.