You must have registered that email address by mistake. That’s not a problem; you just need to sign into AxCrypt using whatever password you think you’ve set. Ensure you type in the exact email address into AxCrypt along with your password.
If you absolutely can’t remember your password then, as a last resort, you can reset your password but this means you’ll lose access to any data that you encrypted with that email address.* It will however let you start using that email address again for newly encrypted files.
*the one you thought you hadn’t registered.