No network after upgrade to 16.04

same here.

I think something went wrong while migrating from upstart to systemd in the installation process?

i will describe how i fixed it

Following this guide i enabled systemd networking: http://xmodulo.com/switch-from-networkmanager-to-systemd-networkd.html

Steps i did:

$ sudo systemctl enable systemd-networkd
$ sudo systemctl enable systemd-resolved
$ sudo systemctl start systemd-resolved
$ sudo rm /etc/resolv.conf
$ sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
$ sudo vi /etc/systemd/network/20-dhcp.network

Content:

[Match]
Name=e*

[Network]
DHCP=yes

Now everything is working fine :-)

Static Address solution:

To add to HerrTaschenbier's answer: If you have a static address on your server you should check to see if the NIC was renamed by doing ifconfig -a I found eth0 was renamed to ens32. Use that information and apply that to HerrTaschenbier's answer. You will also need to create the new file as he instructed, sudo vi /etc/systemd/network/20-ens32.network. The content is slightly different.

[Match] 
Name=ens32

[Network]
DNS=xxx.xxx.xxx.xxx
Domains=somedomain.com
Address=xxx.xxx.xxx
Gateway=xxx.xxx.xxx

Note the file name format has the network adapter name in it. Don't forget to change the adapter name in /etc/network/interfaces file as well. This happened to me from a 14.04 to 16.04 upgrade.