Wifi icon is not displayed

It could be very well possible that your nm-applet application has been killed or crashed.

Try running this command in a terminal:

nohup nm-applet &

if you don't want nohup.out file, you can use

nohup nm-applet > /dev/null &

Run this command from the terminal, just copy and paste for accuracy.

sudo apt-get install --reinstall network-manager network-manager-gnome

This should work if you uninstalled the icon by mistake.

this fixed it for me...

sudo apt-get install indicator-applet-complete