Assuming that users have cleared down their cache and cookies on their browsers, the 500 Internal Server Error is a catch-all error code thrown out when the server is unable for any reason to satisfy a client request. It could be anything, a misconfiguration in the .htaccess file (which tells the server how to handle such requests), a resource timeout (where something in the server takes too long to respond), or issues triggered by incorrect file or directory permissions.
It doesn't necessarily mean that the site is permanently dead, but it's certainly unwell. It may be that someone has been playing with the SMF software on the server and has screwed something up.
The only thing I can suggest is to keep an eye on it. It may well spring into life later on in whatever form.