PAY FOR THEIR MISTAKE

I have a credit card with African Bank, yes it is maxed out but I have never missed a payment, suddenly there's a negative amount on my credit which has changed overnight. I made a few calls to African Bank for some explanation and all I got was that they have mistakenly deposited money in my account for 3 months, now I must pay it back together with my monthly installments and even worse they blocked my credit card 😡