Yes, as Sissy mentioned, please give us more information about your pond, especially we need to know about your water testing results [buy a test kit if you don't already have one]. Please do test your water for ammonia, nitrites, nitrates, and pH [and KH] ASAP and tell us those results.
Do you treat your water with any chemicals?
Do you have any natural animal predators that could have visited the pond?
It's impossible to answer your questions "is it treatable or fatal" until we know more about what's going on. There are way too many factors to consider. For now, what you can do to help the fish is try to isolate/quarantine that fish in a separate tank. Be sure to have aeration [and filtration] and be prepared to do water changes. Don't feed it and don't start it on any medications until you know what is going on. I don't have that info in hand [on dilution strengths], but some of us will add salt to the quarantine water [but look that up before you do anything].
Hope your fish pulls through!