My father-in-law’s Intrepid machine had no ethernet and there was a message: “Wired Network: Device is unmanaged.”. This was strange as it had been working for over 6 months. I still don’t know what happened but I found some ideas here: https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/337685. Open interfaces thus:
sudo gedit /etc/network/interfaces
Edit and save. I just compared my settings with his and suggested he change his to match. NB comment out lines you want to remove, don’t delete them. You might want to look at them later or even restore them.