Here’s what to do if the Ethernet connection on your Mac has stopped working

Apple has posted a support document regarding a recent kernel extension update that rendered the Ethernet port on iMac, MacBook Pro and Mac mini computers inoperable.

Check System Information to find out which version of "Incompatible Kernel Extension Configuration Data” is installed. If you have version 3.28.1, you need an update. If you can connect to WiFi, your Mac will update to version 3.28.2 automatically, or you can follow the steps listed in the support doc to restore it manually.