Going to need more info than that:
Parameters
How old is the tank
What size is the tank
What else is in the tank with the butterfly
How long have you had the butterfly
Have you ever seen it eat
What are you feeding
How often
Do you use any supplements (vitamins)
If the butterfly is new: Was the butterfly QT'd before going in the main tank? Was it eating then?
LoL, OP you really need to give out more info for people to give you more accurate educated guess.
If it's new, butterfly fish take a while to acclimate. If all else fails, try half open clams or buy a small anemone to try to entice it's appetite.
it was qtined and it and some clams and it just stopped eating when i put it in the tank must be my pinktail trigger the tank is 200 gallons it is my dad's it is about 6 years old i used garlic all parameters are normal got it checked out at lfs