A recent update in Linux Mint included the nVidia 367 driver. Even though my HTPC was using integrated graphics, having the driver installed causes Cinnamon to crash with the error message:
Cinnamon just crashed. You are running in fallback mode.
A fix is to remove the nVidia 367 driver completely:
sudo apt-get purge nvidia-367
Reboot and Cinnamon should be running okay again.