Several years ago I hit my upper incisor, and later it became dark. What can I do?

Darkening of an incisor is a sign that the tooth died because of trauma to the pulp during the injury. A dead tooth must be properly treated endodontically, meaning that the dead pulp tissue has been removed from the root canal and the canal filled with a synthetic material. In this way we try to prevent bacteria from developing in the root canal, which could later cause periapical inflammation in the bone (granulomas, cysts).
However, this still does not solve the aesthetic problem. After root canal treatment, a dead tooth usually needs fixed prosthetic treatment: placement of a root post and fabrication of an artificial crown. This protects the tooth against fractures, because dead teeth themselves tolerate biting loads poorly, while at the same time the crown is also an excellent aesthetic solution. Whitening a dead tooth alone usually does not provide satisfactory aesthetic results, and in any case does not reinforce the crown portion as an artificial crown and root post do.

Have a nice day.