Ubuntu 12.04 and later - What to do when network-manager does not start automatically

If you turn on your Ubuntu machine and the network indicator in the system tray does not show up, it's because the network-manager service didn't start.

To fix this issue, modify the /etc/network/interfaces file, comment out all the lines but two following lines:

auto lo
iface lo inet loopback

Reboot, and it's done.