An internal server error is an issue with the server attempting to process requests. When it comes to iThemes Hosting this will most likely be associated with issues within a .htaccess, .user.ini, php.ini, or similar server control file on the account. Typically these files are used to make adjustments to the server's settings to meet the needs of your plugins and the like.
If once renamed your site begins to display properly again you will want to move on to determining what exactly it is in those files that are causing the issue.
PHP Control Files
If the file that you find is a .user.ini or php.ini file these are used to modify the PHP settings of your hosting account. iThemes Hosting uses the .htaccess to modify those settings instead of a .user.ini or php.ini file. So if you need to make any adjustments to the PHP settings you can do so by following these directions.
The quickest way to potentially correct this is to rename any of the above files to something like .user.ini_back or disabledphp.ini anything that will change those names so that they can then be ignored by the server. You can rename those files through your SFTP program.
The .htaccess file is a complicated file that is used to modify the server settings for a great number of server functions. Each individual line within a .htaccess file is used to help set different server settings and adjustments.
The best thing to do to determine if it is indeed your .htaccess file is to once again access your site through your SFTP program and then rename the .htaccess file to something like .htaccess.back. Then create a new .htaccess file in its place with the following required content which you can find here
This should bring your site back online as quickly as possible and then allow you to determine what .htaccess changes you need to make to perform the tasks that you may require from your .htaccess file.
If making the changes recommended above aren't enough or you aren't feeling confident enough to undertake a task like that don't hesitate to reach out to our support team who will be more than happy to take a look at what is going on and help determine what can be done to bring your site back online as quickly as possible.