I upgraded from 13.10 to 14.04 LTS but I can confirm that this problem still happens in 14.04 LTS.
sudo killall systemd-logind -> changed nothing sudo killall NetworkManager --> fixed networking immediately.
Toggling the software "Enable Networking" in the NM applet did not work. Toggling the hardware rfkill switch did not work.
I upgraded from 13.10 to 14.04 LTS but I can confirm that this problem still happens in 14.04 LTS.
sudo killall systemd-logind -> changed nothing
sudo killall NetworkManager --> fixed networking immediately.
Toggling the software "Enable Networking" in the NM applet did not work. Toggling the hardware rfkill switch did not work.