Standard Bank has ****** R2000

I want an immediate explanation and refund of my monies taken from my standard bank account I had a positive balance in my account and then all of a sudden without me using my account at all the next day I was all of a sudden in arrears on my account. I have no overdraft facility and cant utilise my account over my avaliable funds so firstly it is impossible for me to have used funds, secondly my account showed positive balance and then the nexy day Standard bank puts my account in the minus and Im owing almost R2000?