Unauthorized Magtape unpaid fnb

I got a magtape unpaid reversal on my account for my vodacom bill. They took the exact amount needed for my bill . I only had enough in my drafts atm to pay that and now it's been reversed plus a penalty fee !!!! Why would the bank do that ??? I enquired with vodacom and they said its the bank that reversed it not them. Please note I did not request for any type of reversal and I need that phone to be working as it's my main number on my cv which I need in order for potential employers to contact me for a job !! Now I could be Also black listed by vodacom on top of that. This is incredibly unfair!!!! What am I supposed to do now ????