Did you use water conditioner dosed to the size of your tank? Did you temperature-match the water? What are your readings for ammonia, nitrite, nitrate & ph? Some more info will help us to help you better!
You have shocked your Bettas water tempature. It is not the same tempature as before which shocked your betta fish. Too make your betta eat and be active again you can try to change it again. Leave water sitting about 68 degrees out in a bucket. Leave it for 5-8 hours so the water can warm up. Then pour it in the tank. This will be warmer and change your betta back to itself. I did this to my betta too. I hope your betta is okay.
not enough oxygen
same here just changed the water and i have a fish on its side at the top still breathing
@c, Did you properly condition the water before adding it? What brand are you using? Did you temperature match the new water to the tank water? Did you just pour the new water in or gently place it back in the tank? Does your tap have ammonia, nitrite or nitrate in it?