Standard bank messed up my debit order

My home loan debit order usually goes off on the 25th of every month. As nothing happened yesterday. Today I decided to call standard bank to find out what went wrong. Upon inquiry I was told that my bank account number has been changed to the old FNB account number that I closed when I moved to standard bank (my biggest mistake). They cannot even tell me who did and why was it changed. I asked the consultant to change my bank account to my existing standard bank account number and she told me to go to the branch/ I should write a letter otherwise they won't change it. How do they expect me to run around for their mistake? I want standard bank to rectify their mistake so that the debit order can go through ASAP. And also to tell me who changed my bank account and when did i give them instruction to do so. You have already changed the bank account without my consent so why don't you do it again. Make sure I don't get charged any penalties or interest on my account because of your stupid mistake.