Restarted computer during update; "the system network service is not compatible with this version"

I faced the same issue. Workaround is to start the network-manager manually

sudo service network-manager start

Still trying to figure a way to fix it permanently.

Edit: This answer is over six years old and is no longer fresh due to changes in Ubuntu. This is a workaround.

Go into settings, startup items. Add an entry with the following command:

sudo service network-manager start

In a terminal, use visudo to add a NOPASSWD entry for this command so you do not need to give a password:

your-username-here ALL=NOPASSWD: service network-manager start

Next time you log on, this will force a start of the network manager (supposing that it is configured such that it can start up).

I had to reinstall the NetworkManager package and reboot. Everything was fine then.

sudo apt-get --reinstall install network-manager