Last night my iMac’s Ethernet port stopped working. Turns out it seems to have been because of a bad, silent security software update from Apple. Just delightful.
More info here and on this MacRumors thread—the update seems to have been pulled and the fix appears to be to reinstall /System/Library/Extensions/AppleKextExcludeList.kext from a backup. I solved the problem by biting the bullet and installing the 10.11.4 beta.