Obviously common sense will tell you that if your pet his on it's back and not moving there is something very wrong that need to be addressed by a VET not on a forum.
There are several possibilities that could lead to something like that, snake kept in extreme cold temps and froze, over heated snake which would lead to neurological issue which can be temporary or permanent, chemical exposure, near death due to other ailment.
Can you tell us? Age, Size, Enclosure Type, Temperture, How you measure your temps (type of thermomether) etc