How hot is it? I see you're in New York so unless you're heating your house more than normal in the winter I'm guessing it's not heat stroke. Lots of chins lay down, many of them lay on their side when they are sleeping. A chin that is very hot will have pink or red ears provided they are a color whose ears can get red--an ebony chin or a mosaic with dark ears you are not going to see.
Most likely this is NOT an emergency. Keep an ear on your chin and make sure he is eating, drinking pooping. If he seems very lethargic this is a sign of something serious in a chin and needs a vet.