If you are the webmaster of this site please log in to Cpanel and check the Error Logs. You will find the exact reason for this error there.
Common reasons for this error are:
Incorrect file/directory permissions: Above 755.In order files to be processed by the webserver, their permissions have to be equal or below 755. You can update file permissions with a FTP client or through cPanel’s File Manager.
Incorrect Apache directives inside .htaccess file.Make sure you have not specified unsupported directives inside the local .htaccess file. Such include PHP settings and Apache module settings.