Using FTP to regain access to your site's administrator, The failsafe way to regain access to your site's administrator backend is using an FTP application or your hosting control panel's File Manager to rename a file.
Go inside the plugins/system/admintools/admintools directory on your site (on older versions of Admin Tools: plugins/system/admintools). You will see a file named main.php. Rename it to main-disable.php. This will turn disable the Web Application Firewall from executing and you can access your site's back-end again.
After you have fixed the cause of your issue remember to rename main-disable.php back to main.php, otherwise your site will remain unprotected!