Snakes can squeeze themselves through the smallest of spaces imaginable. However, if it is a spare room, with all windows closed, it could have entered from elsewhere in the house and slithered under the door frame (is there a gap?) and stayed there because the room was unused and remained warm.
Either way, Dirk, you need to get a snake expert in and check what kind of snake that is from. If it's venomous, get yourself and the family out of there until the house has been checked over and it found or the house given the all clear.