First, always sudo nginx -t to verify your config files are good.
I ran into the same problem. The reason I had the issue was twofold. First, I had accidentally copied a log file into my site-enabled folder. I deleted the log file and made sure that all the files in sites-enabled were proper nginx site configs. I also noticed two of my virtual hosts were listening for the same domain. So I made sure that each of my virtual hosts had unique domain names.
sudo service nginx restart
Then it worked.