Once I disabled the screen lock using a command that I can't remember.
Now I want to re-enable the screen lock (via Ctrl+Alt+L) but I don't know how.
If I look at System Settings → Brightness and Lock the Lock switcher is dimmed, how to enable it?
Command line you should be able to control this option with...
gsettings set org.gnome.desktop.lockdown disable-lock-screen 'false'
This will lock the screen from command line
If it says command not found you found your answer: you probably removed gnome-screensaver so the option became invalid. (
sudo apt-get install gnome-screensaver to re-install).