Firstly, your dentist should check that the tooth has not devitalised ('died'). This is always a possibility in the scenario of a single front tooth darkening. Temperature sensitivity testing and a radiograph need to be taken.
If the tooth is non-vital ('dead') root canal treatment will need to be carried out to be prevent an acute abscess formation and progression of the darkening.
Assuming that the tooth structure has darkened (and that it is not just superficial staining), in terms of what can be done cosmetically, there are many treatments available, all with their own advantages and disadvantages. Ranging from 'most conservative' option to 'least conservative' options:
1) Vital tooth bleaching
2) Non-vital tooth bleaching (only non-vital teeth)
3) 'Inside/Outside' bleaching (only non-vital teeth)
4) Composite resin veneer
5) Porcelain veneer
6) Full coverage porcelain crown
7) Extraction and prosthetic replacment
If you need any specific information, let me know. However, only your dentist can advise you on which option to take as only they will have the clinical information needed to formalise a diagnosis and suitable treatment options for your individual needs.