I get this problem in the most recent version of Linux Mint (18.1) where no WiFi networks will be shown when I click on the network manager applet. It's very frustrating and until I found a solution, I was left with restarting my laptop to make the network manager work again.
But I found that all I have to do when this occurs is to open up a terminal and run the command:
sudo service network-manager restart
That reloads the network manager applet and the next time it is clicked on, the available wireless networks will be shown.