What did you use to take the chlorine out of the water , I take it your pond is outside yes?
If so did you use cold or warm water to do the water change , I ask this because you can introduce thermal shock by getting the temperatures wrong especially in winter .
Do you have test kits to check your water perameters i;e Ammonia, PH, Nitrite, Nitrate if so what were the reading.
What is the depth of your pond and how many gallons is it , what filtration do you have on the pond, questions questions I know but they have to be asked to help you.
Did you clean out all the dead leaves from the bottom of the pond prior to the onset of winter ?.
When did you last clean out your filter?
Have you treated your koi this winter and if so what did you use , also what is the exact gallonage of water US?
Only by answeing all these questions can we help you
Can you take and post up a photo for us to look at ....
rgrds
Dave