FNB Debited my account without permission

I am so upset with FNB...I have not been working and they debited my account without notifying me for R1600 this may not seem like a lot of money but it is not me as I am now unable to pay my sons fees which could result him being kicked out of the nursery school. FNB deducted money from an account where they have no authority from me to debit!!! I want my money back!!! No one has even called me to make this payment arrangement. Yes I was late to pay but to debit R1600 from someones account without their authorization!!! I want my money back my son needs to go to school. I am recently divorced and this is the only income we have in our home!!! I am soooo disappointed with FNB I need to cut ties with them ASAP!!!!