You usually notice this error due to the lack of a menu bar and all kinds of window decoration after you unsuccessfully played with your Cinnamon settings or just suffered some kind of other error. If you then try to reset Cinnamon or to use any other command involving the desktop environment, you will get the
Unable to open X display
error. While its hard to find out what actually causes it, you can easily bypass it by purging and reinstalling Cinnamon - which is a painless process and leaves your personal Cinnamon configuration untouched. To do so, log in with another desktop environment or open a command line environment by pressing Ctrl + Alt + F1. Afterwards enter
sudo apt-get purge cinnamon && sudo apt-get install cinnamon
Don't forget to log out and in again to make the changes effective. To log out without panel, press Ctrl + Alt + Backspace.