What is wrong Found kio upside down in pond?

I would examine the dead fish and look for damage (Bird or cat attack marks like missing scales etc) and signs of disease (Cotton wool like strands, white spots etc) I'd also do a complete water test and remember to check for deadly poisonous things like Ammonia and Nitrite. If that is not the problem, I would empty the pond completely and clean/disinfect the whole thing and start all over again.