From the description is seems your lower left second-molar has fractured because of gross caries. An Dental X-Ray (IOPA) is recommended to to see the physical extent of damage and to see if some pathology is developing in or around the tooth, followed by a restoration (if possible), or Root Canal Therapy to save the tooth. In rare cases, extraction may also be advised.
Next Steps
Consult a reputed dentist and get an IOPA done, followed by a recommended procedure to save the tooth.
Health Tips
Warm saline rinses may aid to your pain temporarily. Please avoid "over the counter" medicines.