Apple has posted a support document that explains what to do if the Ethernet connection on your Mac stopped working recently.

If the Ethernet connection on your Mac stopped working recently, 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 below to restore it manually.

First, check your version number:
1. While pressing the Option key, click the Apple menu and select System Information.
2. Expand the Software section and select Installations.
3. Click on the Software Name column header to sort the list alphabetically.
4. Look for “Incompatible Kernel Extension Configuration Data.”
5. If the most recent version installed is 3.28.1, then follow the steps below.

If you can connect to WiFi, follow these steps to update to version 3.28.2:
1. Open the Terminal app.
2. Type this command to update to the current version of the “Incompatible Kernel Extension Configuration Data” kernel extension:

sudo softwareupdate –background

3. Quit Terminal and restart your Mac.

If you can’t connect to WiFi, follow these steps to update to version 3.28.2:
1. Follow the instructions to restart your Mac in OS X Recovery. After your Mac restarts, go to step 2.
2. Select Disk Utility from the list of OS X Utilities.
3. Select your drive from the list of internal drives in the sidebar. The default name is “Macintosh HD.” Your drive might have a different name or location, if you renamed or moved it.
4. If the drive name is gray, then your drive might be protected by FileVault. Select File > Unlock from the Disk Utility menu, and enter your FileVault password.
5. Select File > Mount in Disk Utility to mount your drive, if it’s not already mounted.
6. Quit Disk Utility. If you’d like to be able to copy and paste the command required in Step 8, select Get Help Online from the OS X Utilities list to open Safari and view this article on Apple’s support site at support.apple.com/kb/HT6672. Quit Safari before you go to the next step.
7. Launch the Terminal app from Utilities > Terminal.
8. Type (or copy and paste) this command as one line in Terminal. In the example below, the drive name is “Macintosh HD,” and there’s a space between Macintosh and HD. If your Mac’s drive name is different, adjust the text:

rm -rf “/Volumes/Macintosh HD/System/Library/Extensions/AppleKextExcludeList.kext”

9. Quit the Terminal app.
10. Select Restart from the Apple menu.
11. After your Mac restarts, your Ethernet connection should work.
12. Open the Terminal app and type this command to update to the current version of the “Incompatible Kernel Extension Configuration Data” kernel extension:

sudo softwareupdate –background

13. Quit Terminal and restart your Mac.

Alternatively, if you confirmed that your Mac has Incompatible Kernel Extension Configuration Data version 3.28.1, you can also follow the instructions to restart your Mac into OS X Recovery mode, and then select Reinstall OS X. You can reinstall OS X on your drive without reformatting it.

Source: Apple Inc.