If you are the webmaster of this site please log in to Cpaneland check the Error Logs. You will find the exact reason for this errorthere.
Common reasons for this error are:
In order files to be processed by the webserver, theirpermissions have to be equal or below 755. You can update filepermissions with a FTP client or through cPanel's File Manager.
Make sure you have not specified unsupported directives inside thelocal .htaccess file. Such include PHP settings and Apache modulesettings.