NGINX - 502 Bad Gateway

Hi everyone

I have an application running successfully for almost couple of years. But suddenly I am getting the below error. The configuration files are untouched. After then only I tried updating the timeout parameters in config file from the ideas provided through blogs. but no luck. We have used NGINX mainly for reverse proxy, so i hope this is something that needs to be fixed from NGINX (May be IP Blocked. I'm not sure). I have also given below my config file for reference. Kindly guide me in resolving this issue.

worker_processes  1;

events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;

    sendfile        on;
    keepalive_timeout  320;

    server {
        listen       443 ssl;
        server_name  www.xxxyyy.com;
        ssl_certificate C:/nginx/keys/zzzzz.pem;
        ssl_certificate_key C:/nginx/keys/yyyyy.key;
	
        location / {
         proxy_pass   http://127.0.0.1:4446/;
        }

        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   html;
        }
    }


}