Comment 8 for bug 102716

This has happened to me about 3 times in the last month.
ii network-manager 0.6.4-6ubuntu7 network management framework daemon
ii network-manager-gnome 0.6.4-6ubuntu7 network management framework (GNOME frontend

I've noticed that there are some defunct dhcp processes when this happens:
29020 ? 00:00:00 dhclient <defunct>
29927 ? 00:00:00 dhclient3
31366 ? 00:00:00 dhclient <defunct>

This usually helps:
  killall nm-applet
  sudo /etc/dbus-1/event.d/26NetworkManagerDispatcher stop
  sudo /etc/dbus-1/event.d/25NetworkManager stop
  sudo /etc/dbus-1/event.d/25NetworkManager start
  sudo /etc/dbus-1/event.d/26NetworkManagerDispatcher start
  nm-applet &