This problem may be caused by recent PPPoE setting up.
If you didn’t create a PPPoE configure file with pppoeconf, you can make it reveal by
sudo vi /etc/NetworkManager/nm-system-settings.conf
Otherwise, try edit your interfaces configuration file located in /etc/networks/ by
sudo cp /etc/network/interfaces /etc/network/interfaces.bak
sudo vi /etc/network/interfaces
And make sure there are only these two lines included in that file:
iface lo inet loopback
Okay, now restart your computer and you will find your Network Manger Icon appears.