I had to reinstall recently. I installed normal Ubuntu 14.04. I then installed gnome/gnome shell/gdm. I then logged in my user to gnome. Since then I have been configuring my system.
Yesterday, unlock was working fine. Today, the automatic lock screen is not accepting my password as valid. It just takes the input like it's processing it, then takes me right back to the same password prompt. If I click the 'login as a new user' link, it takes me right back to the lock screen. Ctrl+Alt+F1, etc, does nothing.
I had to resort to a hard boot. I've tried this more then once. (first time was from automatic, idle time lock, second time was from my triggering the lock screen with the keyboard command).
I'm certain I have my password right, as I am able to get logged in after the cold boot.
Best Answer
Neither of the other two answers solved the problem for me, but some additional searching led to this bug report:
which solved the issue for me.