NOT HAPPY

FNB allowed a Debit order of R 2256 go off on my account for Medihelp when i did not have sufficient funds in my account putting my account balance in a minus state with almost R 500 i resigned this medical aid and yet the debit order went off. I want this debit order reversed back into my account and my balance brought up to date, how can the bank allow a savings account to go into a minus balance and i am being held responsible for this ??? I want that debit order reversed and the funds back into my account