There could be a number of problems, especially if the animal was wild caught (which most adult box turtles sold at pet stores are). Some never adjust to captivity.
You did not mention what type of turtle you have. Different box turtles require different levels of humidity. Box turtles can get eye and ear infections when not kept humid enough. In the wintertime forced air, which is often used to heat homes, removes humidity from the environment.
You also did not mention what you were feeding the turtle. The reptile could be suffering from a vitamin deficency. In addition, to stay healthy, box turtles need exposure to UV light.
If it is a wild caught animal, it probably is carrying parasites.
At any rate, you should get your turtle to a vet and review your husbandry practices against current literature, such as "Keeping and Breeding Freshwater Turtles" by Russ Gurley.
Good luck!
Tim

Third Eye