https://really-simple-ssl.com/knowledge-base/uninstall-websitebackend-not-accessible/
1 - Open your ftp client, like filezilla for example, and open your website.
2 - Look for the folder wp-content/plugins/really-simple-ssl
3 - Look for the file “force-deactivate.txt”, and rename it to “force-deactivate.php”.
4 - Open your browser, and open the following url: http://www.yourdomain.com/wp-content/plugins/really-simple-ssl/force-deactivate.php (don’t forget to change yourdomain.com in your own domain.
5 - Clear your browser cache, or visit your site with another browser to prevent the browser from redirecting you to https.