You should enable "Force SSL" only when you have SSL enabled on your site
If you have accidentally enabled the option and SSL isn't set up on your server, you won't be able to turn it off because you'll always be redirected to the https:// website URL.
A work around for this is entering the following line in your /includes/config.php:
This will disable the automatic redirection, and allow you to regain access to your admin interface and disable the "Force SSL" setting. After you've disabled the setting, you can remove the line from your config file.