"Layover" or "Sleeping Sickness" as it is sometimes known, is usually a bacterial infection in Koi which is brought about by parasites chewing on the gills or skin. In other instances, it is only a result of severe stress, such as bitter cold or chlorine exposure - The most commonly found parasites are Chilodinella, Costia and or Trichodina.

You have to evaluate for all potential sources of the stress, and correct them. I recommend salting to 0.3% and then if possible, injecting the fish to offset any infection which may be in the system. Several medications are available.