Hello, I can understand your concern. It would have been helpful if you had mentioned which tooth was treated for root canal. As you are mentioning that the tooth was black and had cracks in it,I am assuming that the tooth is in the lower front region and you had had some trauma to the lower jaw area which made the tooth dead and also hurt the chin and lip. Other possibility is that that if the tooth was dead for many years, it may have developed a periapical cyst in relation with the tooth which might be pressing on the mental nerve supplying lower lip and chin and may cause numbness of those areas. Alternatively, the numbness may have started after taking local anesthesia in the lower front area for some tooth problem and the injection might have been given deep. In any case, it seems that the nerve supplying sensation to the lower front area of jaw along with lower lip and chin (mental nerve) is damaged. Usually, if the nerve is damaged, it can regenerate in 3-6 months and the sensations can return. If you are not sure regarding the crown on the tooth, you can avoid it for some months and take care not to bite hard things with that tooth so that it does not fracture. You can get the crown after the numbness is solved. I would also advise you to visit an oral and maxillofacial surgeon for the examination of the area and regarding numbness. He/she might take an x-ray to rule out presence of periapical lesion or cyst in relation with the tooth. Crown should surely be postponed until this possibility is ruled out. I hope this information helps you. Thank you for choosing HealthcareMagic. I wish you feel better soon. Best, Dr. Viraj Shah