I realized that the problem was due to the fact I tried to set up a static connection for my ethernet card via Network Manager's "static configuration.." just before turning the PC off. (The problem appeared when I started the machine back)
The fact that NetworkManager fails to handle the static connection due to the roaming mode is a bug that has widely been reported already. Sorry for the fake alarm. -- [Feisty] Ethernet interface disappeared from Network Manager https://launchpad.net/bugs/85443 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
