I've just upgraded to 14.04.
Whenever I shut the laptop lid and come back to it later, after entering the password the screen just goes blank and requires ctrl+alt+f2
-ing to a terminal to restart lightdm in order to get everything back working again.
I can't see anything obvious in the lightdm logs. I've deleted ~/.Xauthority as per the answer to a similar question to no avail.
Any ideas?