A recent issue has reared up with the Network Manager Applet not loading in Debian and Ubuntu. AFAIK it effects Unity, MATE and Gnome and possibly other Desktop Environments. In all cases the solution is quick and simple.
In your startup add the following command:
Then logout out and log back in, or restart.
This appears to be caused by an update but I have no idea which update package is the culprit. I don't recall an update for Network Manager recently but then I just may not have noticed it.