fnb dispute

On the 25/26 August 2015 i used the Capitec bank atm with my fnb card i withdrew money on the Capitec atm and the atm screen went blank, the money never came out. I then assumed that it was not deducted from my account i did not have my phone with, maybe then i would have seen the R1000 deduction from my account. I used the next atm to withdraw again thats when I discovered that i was short of R1000 in my account . I went to fnb branch to complain and then i was refunded the R1000 and it was credited into my account after 7 working days. To my surprise two months later the money was debited on my fnb account on the 27th October 2015. I don't understand why they debited my account again after it was credited as they gave no explanation and that there was nothing they could do at the branch. Please help me get my money back.