If the chin has recently been very active then it could be that the chin has overheated slightly - this does not always show itself it the ears as many people may have heard.  If the chin has been exercising a lot or stressed then give it a raisin for energy and then leave it to calm down. 

In females, this can often be a sign of pregnancy and often happens in the latter stages.  If there is a chance that your female is pregnant, then whilst she is lying down keep a close eye on her sides and you may be aware of the movement of the kits within her - this generally occurs in the last couple of weeks leading up to the birth.

Check your chins water bottle or watering nipple to ensure that they can get access to the water - occasionally they can block or seal themselves stopping the water from getting through - in these cases the chin quite often will lie down as their muscles will not work effectively if the dehydration is advanced.

Check the room temperature around your chin - overheating CAN kill!  Where possible a temperature of 17 degrees Celsius is recommended though temperature slightly higher than this wont be too troublesome.

If your chin does lie down often then take it along to the vet for a general check over!  It is better to waste money on a healthy chin than not spend it on a sick one!  Chins go down hill very fast once illness does show - don't take a risk!