So you suddenly have no network after you booted Ubuntu today? You did a package update upgrade recently? Well you probably did, even if you don't remember. You probably noticed it when your browser told you it can't connect to the Internet, or by the missing network applet icon in the upper right corner of the screen.
I had this very same issue today, May 2016 (!), although the underlying bug is from January 2016. It's coming from an issue in libnl and makes network-manager not work.
To get to the point (as you're probably reading this on your smartphone, or another machine):
- Get the network up again:
$ sudo ip link set dev eth0 down
$ sudo dhclient eth0
(this works with LAN only, WLAN needs a wpa_supplicant file as it seems, see here)
- Downgrade the buggy packages (don't use the commands from the bug-report, as they'd de-install network-manager) - use this:
$ sudo apt-get install --reinstall libnl-3-200=3.2.21-1 libnl-genl-3-200=3.2.21-1 libnl-route-3-200=3.2.21-1
After that you can take measures to keep this package from updating until the bug is fixed (which should be the case by now, and I don't know why I'm having the issue today..) Make packages sticky, or remove the "trusty proposed" check-mark in package sources.
Now, after the fix, we can talk about all the things which won't work (so people find this post here via search engine):
The small network applet icon is the shortcut to an application called network-manager. It's controlling the network stuff on your Ubuntu. Clicking on it won't work - obviously - when it's gone. So you can alternatively get it by bringing up the Dash and entering "network" or by clicking "System Preferences > Network".
Opening network manager a pop-up will inform you that your version of your network stack is not compatible with this system. ("Die Netzwerkdienste des Systems sind mit dieser Version nicht kompatibel") Good old-skool nonsensical Microsoft wording here... Also, after closing the pop-up, you only options are to configure a Proxy or add a VPN network.
It's not your router. And no, your network cable did not fall off over night.
Don't check /etc/network/interfaces - as it's probably blank anyway (only holding the loop-back interface), and should be this way (and it hasn't been blanked magically by this bug). Network-manager is controlling networking and it's not using this file.
Issuing /etc/init.d/networking restart won't do anything.