Login Screen Does Not Appear

Xubuntu 12.04

after checking the lightdm log (/var/log/lightdm/x-0-greeter.log) if you see the following error:

 "Error opening file /var/lib/lightdm/.Xauthority permission denied"

used the following commands to fix this:

 sudo mv /var/lib/lightdm/.Xauthority /var/lib/lightdm/.Xauthority.bak
 sudo reboot

this will automatically generate a new .Xauthority file which has the correct permissions