Why is NGINX giving 504 gateway timeout error?

My NGINX conf is-

server {
    listen 80;
    server_name site.com ;

    location / {
        include proxy_params;
        proxy_pass http://0.0.0.0:8000;

    }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/site.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/site.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}

I am running a Flask app on Gunicorn on Port 8000. When I try to access my site, I get a 504 Gateway Time-out nginx/1.14.0 (Ubuntu).

In the error.log , it says-

 upstream timed out (110: Connection timed out) while connecting to upstream, client:myip server: site.com, request: "GET / HTTP/1.1", upstream: "http://0.0.0.0:8000/", host: "mysite.com"