By default, Ubuntu disables the root account. If you really, really think you need to log in as root, you could probably use this command to enter a password for root:
sudo passwd root
But the theory that Ubuntu has embraced is that you never need to log in as root. Every administrative thing you need to do can be done with sudo. After using Ubuntu, I have found that I almost always use sudo instead of su on my Mandriva and PCLinuxOS systems now also.