FNB transaction on debt card without approval

Hi,

I have been banking with FNB for a very long time and this is the first time I had a bad experienced. Money was reserved for a purchase from Mr. Delivery on my FNB account which I did not do.After a while the money was refunded but FNB cannot tell me how this transaction of purchase happened, all they say is my details could have been captured from somewhere when I swiped a card.No specific details was given to me and I do not think this is a very professional and safe action from a well established bank. What if this was the last money I had in my account and required it for something important?