First Set the root account password by this method :
go to the terminal and type
$ sudo passwd root
( then u will see the following statements )
Enter new UNIX password :
Retype new UNIX password :
Now after setting the password enable the root account by typing :
$ sudo gedit /etc/gdm/gdm.conf
then a file in gedit opened in the another screen . You have to search the following line :
Allow Root = False
change that false with the true and save it your root account is now enabled.
