Upon starting an automatic rebuild of your dedicated server, you will be asked to input Username, Password and Root Password. The newly created user will be unprivileged and not in the sudoers list. This is done intentionally, with security in mind.
To log in as root, you need to first log in via your user and type the following command:
# su root
You will then be asked for the root password, inputting it in will switch your user to root.