Logstash Unable to push logs to ES

I am collecting logs from a NXlog server & sending it to my Logstash server{ELK Stack on a AWS machine}. It was able to send the logs to ES perfectly but then it just stooped sending logs to ES with the following errors:

{:timestamp=>"2015-10-13T06:41:25.526000+0000", :message=>"Got error to send bulk of actions: localhost:9200 failed to respond", :level=>:error}
{:timestamp=>"2015-10-13T06:41:25.531000+0000", :message=>"Failed to flush outgoing items", 
{:timestamp=>"2015-10-13T06:41:26.538000+0000", :message=>"Got error to send bulk of actions: Connection refused", :level=>:error}

Is this a security group issue? or something else?

Moreover my logstash-output file looks like :

output {
  stdout { }
  elasticsearch { host => "localhost" protocol => "http" port => "9200" }
}