If you have enabled the Security Admin module and entered your own custom access PIN, you'll be presented with the authorisation screen if and when you try to access Shield Security plugin features.
If you happen to have lost or forgotten this, there is nothing that you, or we, can do to help you recover it.
However, you can disable the feature altogether, along with the rest of the Shield features, and either delete your current access PIN or re-enter a new one.
To achieve this, log into your FTP manager for that site and create a "forceoff" file by following the guide outlined in the article here.
And then, you open up the Security Admin module of the plugin once more and delete your current access PIN, or enter/save a new one.
To learn how to delete your current access key, read the article here.
Once you've done this, go back to your FTP manager and remove the "forceoff" file.
Important: If you remove the "forceoff" file before changing settings, the problem will return.
Hint: While you're still logged into your FTP manager, you can also automatically remove the "forceoff" file from the inside of your WordPress site. Just look for the notification at the top of the page you're currently on and click the link provided. The notification will look like this:
Warning - Shield plugin is not currently processing requests
Please delete the "forceoff" file to reactivate the Shield protection. (Click here to automatically delete the file)
For more information about the Security Admin, read the article here.