That is not a system password. That is a SUDO-enabled user password. By default,
Ubuntu does not have a system password. Here is a tutorial on how to fix your
problem.

http://www.psychocats.net/ubuntu/resetpassword

If your system is set up for auto login, I recommend writing your password down.

Once you have recovered control, you can create a system password by typing
"sudo passwd root" at a CLI.