When trying to open EC2 with Putty or transfer file with Filezilla SERVER REFUSE OUR KEY

Given that this is a production issue I'd strongly recommend that you create a support ticket to get assistance. This forum is best-effort and we don't have information about your environment and what is running there.

That said: Have you tried accessing the instance using SSH? You might also try the EC2 Serial Console to see what is happening on the instance - I'm assuming it is Linux but you haven't said.