It's the same issue i had, the gnome lockscreen bug.
Reboot machine to fix it. When your back into your desktop you might try this:
Check your permissions on ~/.Xauthority sounds like root owns it now. You can simply delete it and you should be fine
I copied the file to documents to be sure i don't delete something important. So far i did not ran into lockscreen problems, maybe luck or maybe it's the file.
More details: GNOME lockscreen