get the files corresponding to your server OS from
www.ioncube.com/loaders.php . Then unzip and in there there is a README and test-files, that will test ioncube show you the errors if it's an ioncube config error.
But there is a good chance for something else being wrong:
So, before that, try enabling joomla site debug and check if an error is displayed. If you have access to server logs, please check them too. Otherwise ask your hoster for the hidden error behind the error 500.