It's unclear exactly which automatic update caused the problem, or exactly when it was released—current (unconfirmed) signs point to KB3201845 released on December 9—but whatever it was appeared to break DHCP (Dynamic Host Configuration Protocol), preventing Windows 10 from automatically acquiring an IP address from the network.
There's also little detail on how many people were affected or why, but multiple cases have been confirmed across Europe by many ISPs. A Microsoft spokesperson has meanwhile confirmed that "some customers" had been experiencing "difficulties" getting online, but that's about it for public statements at present.