The symptoms are the computer boots, the desktop environment loads (Gnome, KDE, etc.) and there is no Internet, when there should be. You click on the Network Manager and it tells you "Network Management Disabled." Bug report here. This bug has come and gone for me for a while now.
The problem is an incorrectly configured configuration file. (Basically, this is a bug that should never have happened but it did. Thankfully it's easy to fix.)
Open this file /var/lib/NetworkManager/NetworkManager.state with Admin privileges, and make the following changes. (If you need help on editing a system text file / configuration file, see this Article for a simple New Users guide.)
NetworkingEnabled=trueSave the file and close it. There may be a system service you can restart, but I cannot recall it at this time. Just reboot and your network should return to normally.