Hello again, @Danielle!
I'm sorry for the late reply. Have you tested the external using any diagnostic tool? You should be able to find the brand-specific utility on your HDD manufacturer's official website. There should also be a knowledge base article, explaining how to perform the diagnostic tests.
Unfortunately, this error could be indicating that the HDD migth has failed. If that is indeed the case, then you should definitely check the warranty on the product and send an RMA request to the manufacturer's customer support (if it's still covered). Otherwise, I'd advise you to simply replace the external HDD with a new one and always remember to keep at least one more copy of your data stored on a different drive as well.
As I already mentioned, you would most probably need the assistance of a professional data recovery company if there are important files currently on the external drive that you wish to retrieve. Any data recovery attempts by yourself might cause more harm than good.
Let me know if you have more questions.
SuperSoph_WD