Built-in Ethernet not working
For a very short window (a few hours) - Apple pushed an update that disabled the ethernet driver for some of the Macs.

If you can connect to WiFi and run this command:

sudo softwareupdate --background


That would fix the issue if yours failed to operate due to this bug. The details are covered at


https://support.apple.com/HT205956


Also some more details are on this post:


Ethernet not working on MacBook pro retina 13