Nginx Invalid PID number

nginx -s reload is only used to tell a running nginx process to reload its config. After a stop, you don't have a running nginx process to send a signal to. Just run nginx (possibly with a -c /path/to/config/file)

in my case I solved this by starting the service.

sudo /etc/init.d/nginx start

The command above will start the service in Debian/Ubuntu. It will issue an error if there is any problem (like Apache listening in the same port)

After that nginx -s reload will work like a charm