According to Ubuntu online help
Ubuntu developers made a conscientious
decision to disable the administrative
root account by default in all Ubuntu
installations. This does not mean that
the root account has been deleted or
that it may not be accessed. It merely
has been given a password which
matches no possible encrypted value,
therefore may not log in directly by
If you want to enable root account simply give it some password using
When you enter this command you will be asked for new password. The password you enter will be the password for root account so be careful and choose a strong password.
To disable the root login use
sudo passwd -l root
This will disable the login using root user name i.e your system will be back to previous state.
If you want more information on Ubuntu user management read the official documentation.