Money missing from my account and no response

On the 5th of January, I purchased tickets from a company in the USA. Due to them requiring a code that South African banks don't provide we mutually agreed to void the transaction. FNB had already taken the money from my account and it was in the pending payments section. I expected the money to be returned to my account by the next day. Two weeks later the money is still not in my account and the pending transaction has vanished. My statement shows no payment to the company in the USA. I have lost R2355 and the bank's only response in two weeks is that the transaction was declined due to insufficient funds. This is not true as I had almost R5000 in that account at the time of the transaction. The money has simply vanished. I have sent them proof of the voided transaction and a screenshot from my phone showing the message from the bank that the money had been taken out of my account. I have sent numerous emails and secure messages. The bank obviously couldn't care less. I am short of funds through a bank error or theft on their part and they will not respond.