by default, Ubuntu disables the root account. Since root is godmode, disabling it means that all of the hackers with automated scripts that try to break the root account are wasting their time.
I highly recommend against this. (There are many reasons, here are some: https://askubuntu.com/questions/16178/why-is-it-bad-to-login-as-root) However, the steps to do so are Over on askubuntu.com
According to that page, to unlock the root account you must execute sudo passwd -u root. To relock the root account, use sudo passwd -l root
To reiterate: this is kinda a bad idea. A better solution would be to create a new user that has unlimited permissions ONLY WITHIN THE SCOPE OF WHAT YOU REQUIRE. Root is godmode; if you screw something up too bad for you. An elevated user for your scope, on the other hand, would only be able to destroy what it has access to...not your entire system.