CTRL + ALT + L and CTRL + ALT + DEL are not working in ubuntu 12.04?

I have re installed ubuntu-desktop. After that Ctrl+Alt+L and Ctrl+Alt+Del shortcuts are not working.

If I click on the button on the top right side of the desktop (where things like logout, switch user and shutdown appear) I can see that lock screen has Alt+L next to it, but its colored grey. If I am clicking from there screen lock and logout are working. But the shortcuts are not working.

Solution 1:

As a workaround you could create a shortcut (System Settings -> Keyboard -> Shortcuts tab) and assign it to command gnome-screensaver-command -l

Solution 2:

On Ubuntu 16.04:

System settings > Keyboard > Shortcuts tab > System Change Lock Screen from Ctrl+Alt+L to whatever you want (for example Super+L).