Comment 2 for bug 811524

I have the problem, too. After clicking my user name the user selection window disappears and lightdm freezes showning only the background image. This requires switching to a text console for stopping lightdm and then starting gdm:

sudo stop lightdm
sudo start gdm

Probably one could also start lightdm again at this point and then manually enter user name and password via "Other ..." as the original poster tells.