Comment 21 for bug 851345

What I suspect is happening here is the lightdm daemon has crashed. When it does that CK automatically deletes the ConsoleKit sessions it owned so even though the session and X server continue to run you wont have permission to access things anymore. You can confirm this by running:
$ sudo killall -SIGABRT lightdm
You will have to kill the X server after this and run sudo start lightdm to recover.

The question is, what has caused the lightdm daemon to crash and why hasn't apport restarted it / gone into failsafe X mode?