PostgreSQL server process not running causes connection refusal

PostgreSQL server process is not running, or client is attempting to connect to wrong port/socket location. Default port is 5432, but mismatched configuration causes connection failures.

Check server status with 'sudo service postgresql-12 status' or equivalent systemctl command. Verify server is running and check port configuration in postgresql.conf. If server is down, restart it. If port mismatch, specify correct port in connection string or update client configuration.