Changing your port may help as the 80 port is already being used somewhere
Change your port as below :
listen 8080 default_server;
listen [::]:8080 default_server;
And after that just restart the nginx server as below:
service nginx restart
This will resolve your issue.
In my case the issue was because of the apache server is running somehow. So I just stopped my apache then restart the nginx as below :
sudo /etc/init.d/apache2 stop
sudo systemctl restart nginx
And now it is working like a charm!