FNB took my last cent

I have an arrangement with the bank for a debit order of R1625 from my cheque account. I fell behind on payments due to being unfortunately unemployed for 10 months. FNB not only debited the amount agreed upon but they went into my savings and debit twice the amount. When I asked for a reversal of this error, I am told that because I am behind on my account they will take the money I have with them. I have now no means for basic needs for my family and I because I reserved bread, electricity and transportation money in an FNB account. This is very disappointing after having supporting the bank for more than 30 years. Private banking?? They are so private that they don't even answer the phone