FNB stole my money from my account

FNB took my money(R3,000) from my account (***, trading as; ASL LOGISTICS) for no reason and no one even bothered to call me. when i contacted my contractor since he was the last person to transfer money on to my account (from his account ***, MINT LOGISTICS ANS MOVERS (PTY) ) that he owed me, he told me that his account had been frozen. and he sent me, "FNB Case#***" saying that's the case number FNB gave him. What i don't understand is why would a bank take my money and doesn't even bother to tell me why? Now am scared to even use my account because my money might get taken by the bank. I have lost trust in this bank now because this is too much. I want my money back