Nginx running but not serving

You can verify whether nginx is running and binding to port 80 with something like lsof and/or fstat:

Here's fstat on OpenBSD:

% fstat -u www | fgrep "*:"www      nginx      21404   82* internet stream tcp 0xfffffe833a10d250 *:80www      nginx      21404   83* internet6 stream tcp 0xfffffe833a10d480 *:80

Here's lsof on GNU/Linux:

% lsof -i -n | fgrep nginxnginx    24854        root    6u  IPv4 61955031      0t0  TCP *:http (LISTEN)nginx    25047      nobody    6u  IPv4 61955031      0t0  TCP *:http (LISTEN)

Also, per https://serverfault.com/a/870709/110020, I would recommend against using the sites-available/sites-enabled, as it may also be giving you the extra hassles.

The next step would be to run curl -v from within the box where nginx is running, and/or involve tcpdump to try to find where the packets are getting lost — perhaps you have a firewall in place? Do you get any reply from nginx at all? Is it perhaps your backend that's unavailable?

This is not much of an answer, but my solution was a fresh OS & Nginx install (even though these were already fresh). I'm not sure what was conflicting, but the moment I installed Nginx & copied my configs over, everything worked perfectly.