This is why I have ssh setup to connect remotely from another laptop or something.
First, attempt to keep all the windows and apps running.
pkill -HUP -f "cinnamon --replace"
If this doesn’t work, it means the window manager itself is probably hung and has to be restarted (which will kill all running applications using the window manager).
sudo service lightdm restart
Alternatives include mdm and gdm depending on configuration, but for latest Ubuntu Linux Mint Cinnamon this works given you the displays as if the computer had been restarted.