How can fnb debit an account if I didn't authorise

My savings account was debited by fnb. I didn't authorise this. My loan account was handed to legal. Fair enough. I am under debt review. I advised debt busters that I have been retrenched and upon receiving my payout I will settle my fnb acc in full. I am busy relocating due to my retrenchment. How can I now do this because fnb has debited 3000 from my account. There are no debits on my saving account. I have authorised nothing. I cannot contact them because their contact number is impossible to get through. ***. Why are we being held hostage by fnb? Why is it fine for them to do as they please with our accounts? Where's the fairness? I want that debit reversed. It has now created even more financial stress than ever. Not to mention emotional worry.