Keyboard shortcut for "Lock screen" not working

In the top panel right corner choose System Settings to select Keyboard in the Hardware section, or type "key.." in the Dash then select Keyboard:

In the Shortcuts tab all key combinations are listed. You may change them there or add your own custom shortcut. To have the poweroff shortcut back add it with a custom shortcut for the following command:

gnome-session-quit --power-off

Note: to be able to add a custom shortcut we may have to clear an already existing shortcut if this uses the same keys.

The GNOME lock screen feature depends on the gnome-screensaver . In case you have removed it or did not install it in a custom environment you will have to install it first.

This seems to have been improved in 12.10.

If you go into System Settings > Keyboard > Shortcuts, System,

then left-click once on 'Lock screen',

the Ctrl+Alt+L on the right changes to 'New accelerator...[something]', at which point you can hold Super [Windows key] and press L.

The keyboard shortcut will then show as 'Super+L' and works immediately.

I was experiencing this symptom, but none of the above solutions were working. I found out that this was because I had previously disabled the screen lock because I did not want the screen to lock when the screensaver starts.

The takeaway is that screen lock will not work unless it is enabled in your gnome settings. You can ensure it is enabled with the following command:

# Note the disable flag is set to false, so its really enabling the behavior
gsettings set org.gnome.desktop.lockdown disable-lock-screen 'false'

After running this, the Super+L hotkey worked fine.