Sounds like you have the problem discussed here:
https://bugs.launchpad.net/ubuntu/+s...er/+bug/869635
Open a terminal window & enter the following command:
sudo gedit /etc/init/network-manager.conf
Add the following line as per below: stop on runlevel [06]
It should now look like this (that's an arrow so don't include it):
start on (local-filesystems
and started dbus)
stop on stopping dbus
stop on runlevel [06] <--------------------------
expect fork
respawn