I have been having this trouble since I started to use my Internet connection using pppoeconf package. The thing is that, pppoeconf turns out to disable or override all other wired network.
This is the way to get it back in place:
1. Fire a terminal.
2. Type sudo gedit /etc/NetworkManager/nm-system-settings.conf
3. Now you should see something like this:
4. Change ‘false’ to ‘true’
6. Comment this post, whether this helped you or not.