Hello. The lower left tooth seems to have a deep cavity. From this OPG it appears to have affected the innermost chamber of the tooth that contains the nerves and blood vessels. Once you visit a Dentist, the decayed portion of the tooth would be cleaned and checked if the cavity has reached the pulp chamber.
If it has, then an RCT is a must.
Next Steps
Visit a Dentist as soon as possible
Health Tips
Do not chew on left side.
Do not press or cover the face on left side.
DO NOT DELAY IN VISITING A DENTIST.