Hole (sinus) is a sign of infection; the reason for that is deep caries, periodontitis ( pierrhia), or Combination of that( endoperio lesion). In this condition, root canal treatment, if required than surgical cleaning or if not curable than extraction (removal) of tooth.
Next Steps
radiographs to diagnose them at dental clinic
Health Tips
warm saline rinses for every two hours will relief from this condition.