Well you should have a secondary user other than root on the system to login with. Hopefully that user is in the sudoers file and has root permissions or is a part of the wheel group?
If the above criteria is met you can do the following from terminal:
sudo vim /etc/pam.d/gdm
Then comment out this line:
#auth required pam_succeed_if.so user != root quiet
Then open this file:
sudo vim /etc/pam.d/gdm-password
Then comment the following line (same as the above):
#auth required pam_succeed_if.so user != root quiet
Log out of that account and login in as root using the "Other" option. You will get some long winded alert about how it is not secure to use the root account, just close and you are in!
http://superuser.com/questions/209865/enable-root-user-in-gnome-for-fedora-14