Unauthorized debit order, I want all my money back!!!!!!!

I paid up my personal loan but FNB still debited it yesterday and that resulted my account to be overdrawn & charged overdraft interest. I went to my nearest branch this morning & on the phone I was told the system was down but the manager helped me to lodge a complaint. Now I'm checking my bank balance there is money credited into my account but it's the amount that was debited. They also didn't refund me for the interest the charged me for the overdraft. I want my money back now!!!! On the app the balance is ZERO for the personal loan so where did my money go & why is it short. This is an unauthorized debit order. I want my money back with the interest you took from the overdraft!!!!!