This error happens when your web server is unable to process your request because it's overloaded. Everything should resume normally once the load goes down. This error is also used during maintenance. This error also appears when all backend servers for a load balancer are offline and none of them can process your request.
How to fix:
If you operate this server, make sure your web server is not overloaded and has enough resources to process requests. Also, make sure your backends are all up and running fine.