The "Settings" button has disappeared from Software & Updates and it's no longer found under System Settings. If I run the Software Updater from the Dash it tells me that the Software on this computer is up to date. If I press OK it closes. There is no "Settings" option. I can't figure out how to get into Ubuntu Software, Other Software, Updates, Authentication or Additional Drivers. I'm running Ubuntu 16.04.01 64 bit. While searching for a solution, I found the recommendation that I remove and reinstall the Unity Control Centre, but that hasn't helped. I am an Administrator, so that shouldn't be the problem.
I found the answer: sudo apt-get install ubuntu-desktop This gave me the following:
The following NEW packages will be installed: apturl apturl-common nautilus-share software-properties-gtk ubuntu-desktop
I have no idea how these got deleted, but they're back now.