Having identify the plugin causing the issue with your site, you need to uninstall it in order to return everything back to normal.
Since you can currently not access your site but WordPress administration dashboard as well, you can still uninstall the really simple ssl plugin either through your cPanel, FTP, or database.
If you do not have access to your WordPress installation through those means, you should contact your hosting provider or website administrator; they should be able to assist you with that request.
Uninstall a WordPress plugin through cPanel or FTP, in two (2) easy steps:
- Navigate to your WordPress installation, then to your
wp-content/plugins/directory. - Once there, you will see all your installed plugins. In order to uninstall the
really simple ssl, all you have to do is to simply rename (to something like:really simple ssl old).