Who are you supposed to contact when you see a HTTP 500 error?
Server errors are just that - errors that occurred with your web hosting server. You can Google error codes for an idea of what's going on, for example: Error 500
Since you're able to reach the domain, it's not an issue for the domain registrar - contact your web hosting company's support team for help.
If you have access to your web hosting server logs (look under the directory "logs" using FTP or a Filemanager script), you can review the error log for additional information as to what occurred.