Discoloration can be due to various causes. Prolonged staining of teeth would also make it appear as brown. Also tooth decay might also make it appear as brownish.
Next Steps
Please visit your nearest dentist for a full mouth check up to find out the actual cause for the discoloration and follow appropriate treatment.
Health Tips
If it's symptomatic (sensitivity/pain), please don't ignore it for long. Neglect will only increase the cost of treatment.