System settings is missing. I tried reinstall it as it mentioned here, but it seems to be installed and deactivated. This problem occurred when I installed Elementary desktop environment and uninstalled it afterwards.
You can try to reinstall
gnome-control-center from terminal:
sudo apt-get update sudo apt-get install --reinstall gnome-control-center
In case you install Gnome 3.12 on Ubuntu 14.04 via the gnome3-staging ppa you need to:
sudo apt-get install gnome-settings-daemon-schemas
This will install the settings daemon correctly. There seems to be an issue with dependencies and correct upgrade paths.
After you do this, log out and back in, then go to (search your computer and online sources) top left type in (system settings) then drag the icon to your launch bar. That will put it back in the left launch bar also