Unacceptable Service- Money disappearing from my accountb

I received a tax refund from SARS yesterday, to my unpleasant surprise and utter shock, today the refund amount disappeared and my balance is R 0. Upon calling FNB I was informed that the bank has reserved my refund until such time SARS confirms the refund. I received no form of communication from FNB in this regard. This is a disability tax rebate and I urgently need the funds for medical suppliers. This is the first time experiencing such unacceptable service from a bank. I'm really considering returning to my previous bank. I'm so inconvenienced. FNB, please release my refund!!!