Tooth discoloration is because of loss of vitality of the pulp leading to blackish colour of the tooth .
Solution-
1.visit dentist & get tooth xray (iopa)&tooth tests done (as part of investigation).
2.get the treatment done accordingly as per the investigations i.e Root canal treatment or extraction depending on the amount of decay , condition of the remaining tooth structure ,status of the surrounding tooth structure i.e periodontium etc.
Next Steps
get the treatment done at the earliest
Health Tips
don't worry .It has solution with proper expertise & technology .
Thanks!