It sounds like the koi was electrocuted. Do you have a submersible pump or underwater lights? If so...check there first. If not, it may have been a lightning strike. Lightning strikes and electrocution can affect just a single fish in the pond and leave the rest unharmed. If the fish can't swim and eat, it will start to go downhill and eventually die. There is no cure for his condition because there is no way you can straighten his back out. I think as long as the fish was still doing OK with the rest of the fish I'd leave it alone. But if the fish is already spending his time sitting on the bottom, unable to swim and feed normally, it might be time to euthanize him. 16 to 20 drops of clove oil per gallon of water is a humane way to do it. Mix the clove oil in well, place the fish in and just leave him until he is dead.

there are a couple other possible causes for kinked back, including overdose with organophosphates and lack of vitamin C (not in a big fish though) and trauma (such as improper netting and carrying).

Just to be sure, could you post a picture of him? I'd hate to suggest euthanasia without seeing how bad off he really is. I do with I had better news for you. Here's a pretty good link that explains more and has a few photos.

Here is a link that might be useful: kinked back fish

All the pumps and lights are external. We did have an issue in the spring with a water spike in temperature and lost about 20 babies from last year. We put up a shade sail and we quickly corected the issue and all was well. We only lost the small fish all less than 2 inches. We did have numerous thunder and lightning storms over a summer with record breaking high temperatures. Wouldn't an electrocution affect more than one fish? The picture on the website look a lot like our fish. How can you tell if it is suffering? We are in a quandary about whether to euthanize our pet.

Lightning can indeed affect only one fish. For some strange reason, it usually only affects one. I am not sure I have ever seen a case where lightning affected multiple fish. It's strange, isn't it? At any rate, since the fish is already a big one, chances of recovery are pretty much non existant. I think if it were one of mine, I'd euthanize him before winter comes. Cold water stresses even healthy fish. One that is isolating its self and is unable to swim with the others will be even more stressed. Plus he is probably not able to eat sufficient amounts to maintain a healthy body weight. If you chose to wait, he may adjust and learn how to swim more, but my guess is he won't. Resting on the bottom will eventully lead to sores on his underside, and then infection and death anyway. So sorry for the bad news.

One thing that puzzles me. You said that during your temperature spike you lost only small fish. Did you test your water during this period? Usually higher temps cause oxygen problems, which affect large fish first, not small ones. when smaller ones die, I suspect nitrite problems. Have you tested for ammonia and nitrites? Not that those things would have caused the problem you are seeing with this fish.