I have ubuntu 12.04, on a Dell Inspiron 14r, Intel HD 3000. I set on the Power settings, "When the lid is closed: Do nothing." The password prompt worked just fine when i closed and opened the lid.
But now after i have updated the Intel graphics driver, most of the times, when i open the laptop lid, there's no password prompt. I've tried typing the password blindly, but it does not respond to keyboard or mouse input.
The only way out of it, is to use tty1, log and restart lightdm with the following command:
sudo service lightdm restart
But the problem is, it closes all my previous opened programs.