Error: The remote server returned on error: (503) Server Unavailable
The '503' error usually means the Foundation site is down.
Navigate to your Foundation site through a browser window.
If you are unable to reach the site through your browser, reach out to the internal team responsible for Foundation.
If you are able to reach site but not composer then go to the Server where composer or rest of components are installed and restart the Pool and also make sure the rest of the pools are running too.
In case of the Error from Studio ; if Foundation is running ; still perform and IIS Reset make sure the App pool is running ; then check the scenario.
If issue doesnt get resolved share the application trace.