This is most likely a case of non vital tooth secondary to trauma. Ideally such teeth are treated by root canal treatment followed by a crown which takes care of the discolouration that has taken place, but if the tooth is asymptomatic with no signs of active infection and the only thing that concerns you is the change in colour you can opt to get a tooth coloured Composite layering done which will mask the discolouration.

Is the entire tooth discolored or only one particular area of the tooth as you mentioned. Get the vitality test for the tooth done. Get an Xray done for the tooth and proceed according to the treatment plan If it's a cavity only - cosmetic restoration will help,

Go for Rct f/b bleaching of the discolored tooth to match with rest ! If you are happy with tooth shape auze appeaeance ! It wont require crown or veneer unless substantial tooth structure is lost

Internal resorption has taken place with the tooth due to injury...please see an endodontist...they will check weather tooth is vital or dead...go ahead with root canal treatment and crown placement if you want a permanent solution...

It may be a non vital tooth. Get an X-ray done to check. Only after that your dentist can decide if bleaching will work for you or not. But non vital tooth should be root canaled and then you may opt for a zirconia crown to restore your smile