If you have enabled the Security Admin module and entered your own custom access key, you'll be presented with the following 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 re-enter a new access key.
To achieve this, log into your FTP manager and create a "forceoff" file by following the guide outlined in the article here.
Once you have done this, you open up the Security Admin module of the plugin once more and enter/save a new access key.
After entering/saving a new access key, go back to your FTP manager and remove the "forceoff" file.
For more information on the Security Admin, read the article here.