Lock Screen not working in 12.04

[drbono]

If you have Cinnamon installed (even if you are using unity) and you recently got the 1.8 version update, it uninstalls gnome screensaver. If you reinstall gnome screensaver, logout and back in...worked for me.

[OrangeVixen]

I believe what is happening here is a conflict between gnome-screensaver and xscreensaver.

By default when you install Ubuntu 12.04 Precise, you get gnome-screensaver. However all gnome-screensaver does is blank the screen (or lock the screen) to black.

xscreensaver on the other hand has a lot more "eye candy", however it is not integrated to the GNOME settings and you can not lock it from the Session Menu (the menu item at the upper right corner of the panel).

Further, xscreensaver conflicts with gnome-screensaver (even though the packages do not seem to indicate it). so you need to check (run Synaptic) and see if you have both installed. You can check by running Synaptic and type in "screensaver" in the Quick Filter.

You should only have one installed, there is this HOWTO: Replace gnome-screensaver with xscreensaver http://ubuntuforums.org/showthread.php?t=195557