Is ubuntu complains for a pass probably this is user's pass, cause in Ubuntu by default 1st user's a/c is an admin, To reset pass it is needed to boot into recovery mode by pressing shift key at the boot time. Now select Remount /Read/Write.....other filesystems and again select Root drop to....shell prompt. Here u may see the root prompt where u simply write passwd and press enter..... after type ur desire pass and retype the same again type sync and then type reboot -f

hope it helps u !

If you can login as root, you can open a terminal and run:
passwd
where is the actual username of the account whos password needs resetting. It will prompt you to create a new password.