Comment 30 for bug 1159707

I had this with Linux Mint 17.2 yesterday, updated to 17.3, and had the problem again today.

However, I fixed it by (without losing the session):
    Swapping to another user (ie another virtual terminal)
    sudo -s
    ps -edf | grep "cinnamon --replace"
    finding the pid of "cinnamon --replace" for *my* user (ie, the hung session)
    Killing that pid:
         kill <pid>
    Swapping back to my user (the previously hung session)
    clicking on a terminal window (not root)
    entering:
         cinnamon --replace
Everything was back to normal. :-)

This only help if you have/can get a terminal window!
Hope this helps!