@Lewis-Adams when you added the subdomain did you make sure to edit the url in config.json and on NGINX?
as @scottalanmiller mentioned this will be the main cause for the disconnection
Ehmm, thats a unique one. Did you check your apache/nginx settings? Perhaps they got wiped when you did something related to your server?
@UncleSam Check your DNS settings. This has nothing to do with NodeBB. Your domain doesn't seem to be pointing to the IP address. Pinging your domain is not resolving to the IP address.