My server is running great yesterday but now it returned a 502 error, how could this happen?

In my access.log shows:

[24/Aug/2016:07:40:29 +0000] "GET /ad/image/414 HTTP/1.1" 502 583 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36"

In my error.log shows:

2016/08/24 07:38:50 [error] 14465#0: *12513 connect() failed (111: Connection refused) while connecting to upstream, client: 123.49.616.74, server: app.example.com.au, request: "GET /ad/image/414 HTTP/1.1", upstream: "http://127.0.0.1:8000/ad/image/414", host: "app.example.com.au"

This is the result when I run grep:

ubuntu    6856  0.0  0.6  56624 12652 ?        S    00:08   0:03 /home/www/carbuzzz_admin/env/bin/python /home/www/carbuzzz_admin/env/bin/gunicorn application:application -b localhost:4215
ubuntu    6865  0.0  1.8 180832 36892 ?        S    00:08   0:00 /home/www/carbuzzz_admin/env/bin/python /home/www/carbuzzz_admin/env/bin/gunicorn application:application -b localhost:4215
ubuntu   14300  0.0  0.0  10464   948 pts/0    S+   08:06   0:00 grep --color=auto -i gunicorn

The HTTP status code 502 normally means that the application server Nginx is proxying to is not running. As this is a Flask app I presume this is something like Gunicorn or uwsgi. Check to see if that is running.

I normally use something like Supervisor to ensure that the application server gets restarted automatically if it goes down.