This error happen for a wide variety of reasons. Normally, this error can be due to a temporary overloading or maintenance being performed on the server and it is resolved after a period of time or once another thread has been released by web-server application. In most cases this could happen (assuming there are no faults in your app) if there are long running tasks and as a result the request queue is backed up.The subsequent points serve as a possible fix, aimed toward resolving the 503 Service Unavailable Error.
- Reload (Refresh) the page
- Scan for Malware
- Visiting the website later
- Contact server admin