the probable reasons of one tooth turning brown could be after some impact hit on the tooth .its also called concussions or intrusion in such conditions the tooh may turn brown to grey and usuall some pain or some small swelling also this has to be treated conservative. by medication and observation then if required it may be treated surgical by pulp therapy or such meyhods I will advice you must see a dentist as emergency because you have delayed quite a bit the other reason to become brown is a decay in the tooth and requires attention