Maybe this is a temporary issue which should be resolved after you restart the computer.
If your computer doesn't recognize ethernet cables, it is possible that the network card/adapter (or Ethernet) is disabled.
If you are not sure how to enable it, check out the don't know how to enable/disable the network adapter solutions to learn how to enable the network card/adapter.
If the computer's Ethernet port is dirty, it won't detect the Ethernet cable you plugged in. All you need to do is to take a small piece of cotton (or a cotton bud), pour some isopropyl (rubbing) alcohol on it and clean the Ethernet port.
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.