I've installed NGINX, GUNICORN and my project properly on Ubuntu server, 
but when I run the project using
python manage.py runserver, I get the following error; 

django.db.utils.OperationalError: (2003, "Can't connect to MySQL server on '127.0.0.1' (111)"

Before, installing gunicorn, my site was running properly at my_public_ip_address:8000
check your mysql server is running or not
restart the mysql server
django.db.utils.OperationalError: (2003, "Can't connect to MySQL server on '127.0.0.1' (111)"