If you find that your Ethernet port has stopped working, it is because of a ‘silent’ update from Apple in the last week of February, which Apple have now fixed.
A kernal extension corruption
To do the following you’ll have to connect to your WiFi network….
Hold the ‘Option’ (Left Alt) key down and then click on the Apple logo in the top left of your screen.
Click on System Information, and in the left hand navigation window scroll down to Software (click on the triangle to expand it, it you have to), and then click on Installations.
It should be in alphabetical order, but if not then click on the Software Name in the right hand window. Scroll down till you find ‘Incompatible Kernal Extension Configuration Data’. There will probably be three listings from different dates. The last one should be version 3.28.2 and have a date from the 28th February onwards. If you have version 3.28.1 then you will need to update to the latest 3.28.2
Open Terminal (which is in Applications > Utilities)
Then type in the following command below exactly as you see it, followed by your admin password. You won’t see your password as it is permanently hidden, so don’t worry.
sudo softwareupdate –background
Quit Terminal and restart your computer.
Another way is to boot to Recovery Console, by holding Cmd R when booting the computer and then reinstalling your OS. The Terminal route is quicker and assumes you have access to a WiFi network to get your Ethernet working again.