I want to know if it is possible that if my NGINX send a request to Unicorn and NGINX doesn't receive an answer, can NGINX send the request to Unicorn again?
Not normally, no. I can think of a workaround, but I'm sure that I would advise it. Using the Nginx upstream module you can define multiple upstream backend ends. Nginx has no way of knowing if the the different upstream servers listed are actually the same backend, nor does it care. The docs for the upstream module say: " If an error occurs during communication with a server, the request will be passed to the next server."
So if you arrange to have your app available at two different addresses (sockets, domain names, IPs or ports), then you can set Nginx to use the upstream module to try the same request twice.
If the the app was not responding properly in time the first time the request was made, there's a good chance it won't respond successfully on the follow-up request. Using the upstream module in the intended way is recommended instead: Use two unique app servers, who might be sharing state through a backend resource like a database.