First try to boot to init 3.
If you have a .Xauthority file in your users account, rename it to .Xauthority.bak.
Then as root type init 5 to restart your KDM/GDM/XDM session manager.

Basically, the .Xauthority file was no good and it needed to recreate a new one. If you're trying to log in X as root, you will have to rename root's .Xauthority file as well. Once you log out and log in as regular user you might have to export the .Xauthority file to root's account from a terminal as root.

HTH,

Durand