By default, Ubuntu disables the root account for logins. Sudo is used instead.
You can run a command with root privilleges by adding sudo before the command. Eg:
will open /etc/fstab in vi with root privilleges.
You can get a shell with root privilleges using:
The prompt will change from $ to #, and any commands you enter will be executed with root privilleges.
In both cases, you will be prompted to enter your users password.
If you really want to login as root (it is not necessary, sudo does everything you need), use the command:
This will prompt you for a password for root, and you can then login as root.
I hope this helps
--Ian