Comment 16 for bug 1307545

For all those who have no success editing /etc/systemd/logind.conf or found that rm ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-power-manager.xml only works temporarily, I had success using gksudo gedit /etc/UPower/UPower.conf and then setting IgnoreLid=true.

A second work around is to hit ctrl+alt+f1, sign in using your username(lower-case) and password, then enter sudo restart lightdm, followed by your password again (some times this has to be done all with a pitch black screen)

Not ideal I know but is the best solution i've found.