Low oxygen or poor water quality can make fish go off feed without killing them. If you can look at your pond at daybreak you might see them at the top gasping for air.
Side note slightly related: I have a small bass in an aquarium on feed that one day was sitting on bottom and wouldn't eat. Next day coming to top to look at food and today acting hungry but not eating. The water is pretty clean. I guess fish get sick sometimes too. Maybe your's will recover.
