FNB debited my account without my permisson!!

FNB decided to deduct a whopping R8000 off my account today without my permission. I called them and you cannot get through to reverse the debit order as this function is apparently unavailable. Trying to chat to someone on Secure Chat but they said they would direct my message to someone and now there is silence. This is unacceptable and I want this debit order reversed IMMEDIATELY! This is ridiculous FNB. What kind of thinking goes into something like this?