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. 

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