My account hacked

I had about R985 in my account. On the 3rd of October I woke to an sms e for R899 which in itself is ridiculous because I have never bout airtime above R120 with my cellphone banking. I went to the bank and they said they will investigate and get back to me which they have not done as yet. I contacted Vodacom as my service provider and they have no evidence on their system that any airtime of that amount being loaded onto my account. I am very unhappy with Nedbank because they have not followed up nor have they made efforts to contact me. This was is a lot of money and I am merely a student, I am appalled at how slow the service and investigation has been. I hold the bank responsible as when I went in they could not give me any information and it is unfortunate that as a customer I am not being taken seriously.