FNB takes my money

I had a problem with FNB in November and only after I had contacted you did they refund me. R18,00 was taken from my account then when I was in Mauritius. Now I am back in Mauritius again. Yesterday for the first time I withdraw Rs1,000 Rupee. I knew I had no more money in my account . I was waiting for money from hotels. I had Rs802.81 Rupee left. Got a letter today from FNB to say I am in arrears. Over drawn with almost R12,000. Somehow they let R12,000 out of my account that has NO money in it. No overdraft for that amount. I got the funds from the hotel, but cannot clear the Forex because then FNB will take it for the overdraft. FNB did it once again . Taken money from my account without me getting it. Please help