I had the same problem yesterday night, this occurs when webserver is shut down or busy.The error will automatically go away once the webserver back to normal.
Few downtimes are common even shared and vps providers experience downtime but thats not a big issue at all. It is up and running fine now so dont need further discussion.
As I said the problem will happen if the webserver is not functioning normally.
We can check whether all are facing the same downtime by using services like downuptime.net, websitedown.info etc.