How to lock my screen?

I can't enable lock screen mode in my laptop and also in my PC. I went to system setting > screen > lock Ctrl+Alt+L : Nothing happens! the shortcut is there but not works !

sudo apt-get --reinstall install gnome-screensaver :
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
 dassault-systemes-draftsight:i386 : Depends: sendmail:i386 but it is not installable
                                     PreDepends: libdirectfb-extra:i386 (>= 1.2.7-2) but it is not going to be installed
                                     PreDepends: libxcb-render-util0:i386 (>= 0.3.6-1) but it is not installable
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

Solution 1:

Ctrl+Alt+L

The above keyboard shortcut will lock your screen.

Solution 2:

I installed ubuntu tweak . go to the "Tweaks" part and then to the "Security related" . Then unchecked "Disable lock screen " ! Thats it !

Solution 3:

I was reading around & saw that some computers if CON+ALT+L doesn't work to try the "Windows" botton + L (the button with the little Windows logo, if that's the type of computer you're using) WIN+L ---> It worked on mine, I couldn't find the shortcuts mentioned above so I tried this & it worked! Good luck!!

Solution 4:

Try screensaver command, and see if this is the keyboard (shortcut) or the application (graphics function) that is not working:

gnome-screensaver-command -l

Also check shortcut: (should be Ctrl-Alt-L )

>> System Settings >> Keyboard >> Shortcuts >> System

  Lock screen                                  Ctrl+Alt+l