Fortunately, I had a file on my desktop. I could alternate-click on it and use the context menu to open it with my internet browser.
This page showed me how to check if the Unity application was installed. Open a terminal with CTRL-ALT-T and then use internet browser copy (CTRL-C) and terminal paste (CTRL-SHIFT-V) to run the following commands.
(The "sudo" command will ask for your password.)
sudo apt-get install compizconfig-settings-manager
Since I had no Unity application in the resulting list of displayed applications, I went to this page for how to reinstall the missing Unity application.
sudo apt-get install unity