The 'root' user does not have a password and therefore you cannot login as
that user. More importantly, are you trying to log in to Gnome with the
'root' user? Because you should not do that.
You have to log in with your own user account and if you want to execute a
command as 'root' you can either run
sudo somecommand
from the command line. Or if you want to run a GUI application as root you
can press ALT+F2 and enter:
gksudo somecommand
Please read about why Ubuntu (and derivatives) do not use the root user:
https://help.ubuntu.com/community/RootSudo
