I don't want you to have to wait any longer, so I'm going to go ahead and answer as best I can without knowing the additional information.
From what you have described, I believe your koi went into shock. There must have been some kind of sudden change between the old water and the new water. It may have been colder, which results in shock. As water stays in a pond the PH changes. Then when you change all the water, the pH goes back to whatever comes out of the tap. If that's a drop, pH shock results. Not getting all the chlorine out can also put a fish into shock. Sometimes, just being netted and moved around can do it.
Unfortunately, once a fish goes into shock, it is hard to save them. Don't add medications as that can make the situation worse. One of the best treatments for shock is an injection of a corticosteroid. Dexamethasone is often used with koi. You would have to contact an aquatic vet who makes house calls (many of them do). This link will take you to a directory of aquatic vets:
If you can't consult a vet, add pond salt at the rate of one teaspoon per gallon of water as a good first aid measure. You can also increase the aeration to help the fish. Temporarily set up a second pump with a bubbler or fountain. Don't change the water temperature again, as that just causes more shock. The water will gradually warm to what it was before. If you have a way to keep the fish moving, that will get more oxygen circulating through him. Don't feed him. These measures will be all of the first aid you can do on your own.
If you have more questions, just let me know. I hope your koi will be able to recover.
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