It's possible to accidentally lock yourself out of the administrator area, especially when using the Exclusive Allow IP List or IP Disallow List options of the Web Application Firewall. The easiest way to work around this issue is using an FTP application or your hosting control panel's File Manager to rename a file.

Go inside the plugins/system/admintools/services directory on your site. You will see a file named provider.php. Rename it to provider-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 provider-disable.php back to provider.php, otherwise your site will remain unprotected!