Audible can sometimes lock your account if it detects any suspicious activity. You will not be given any specific reason for locking your account but it can be that they were unable to charge your credit card, billing issues, or your IP address triggered some unusual activity.
You can contact customer service to request for your account to be unlocked. Audible requires that you log in to your account for the issue to be resolved. However, this is not possible because the account is already locked.
You can try to contact support via Twitter @audiblesupport or Reddit @audible_com.