Both nginx and Symfony produce logs:
Symfony: [your project directory]/var/logs/dev.log or prod.log
If you're able to run the default page, I think that Nginx works, and the problem is on the framework side. Routing problems are generally reported in Symfony log files and they should give you some clue. For development purpose, use app_dev.php file instead of app.php. In the development mode, Symfony shows detailed information about exceptions directly in the browser, and you have an access to the profiler, too.