Money dissappearing in my credit card. Transactions they can't account for.

I recently made a payment into my credit account as so I do monthly. But then I was called by the bank that I haven't made my payment. When I look through my transactions it was like it evaporated. R3000 just disappeared and it could not be traced by any transaction by me. The bank said they are perfect their system could never take in money without my knowledge. I was puzzled because I know I had put that money to pay. In the end they said I made a mistake.... I must've thought I put that amount in. So my question is where the rest could be? , because I surely was never able to spend it!!