This error is normally only temporary and may resolve itself. It means exactly what it says in that the server is temporarily unable to handle the request, although it is functioning as normal otherwise.
In cases like this, make sure that your scripts, software, plugins, themes are functional. Common root causes are poorly coded software such as plugins and themes, so if you've installed anything new, you may have to remove it.
Has the traffic to your site suddenly spiked? If your system has come under a heavy traffic load, that can prevent the server from properly serving all incoming requests. The server itself has not actually crashed or shut down but has instead set itself to a mode of service that will prevent any additional requests from being served as normal.
It's likely something has stopped working correctly, or you're simply running out of resources. In an event such as this, you may want to consider upgrading your resources.
If you can't seem to find anything wrong and it doesn't go away, please contact our technical support team to help you get to the bottom of it.