money out of my nedbank account

I've realised this morning that their was a payment made to the amount of R5610.10 that I did not authorise in any way or matter. Upon investigation I found out it was a stop order which immediately made the warning lights go off. I NEVER signed any documents supporting this! I pay a fee to keep my money safe and yet they dish it out to anybody? They couldn't supply me with any signed documents, voice recordings, court orders, anything that they based this decision on as yet........what do you call this? Theft? *****? a Nasty mistake??? will this happen again? When?? to whom?? And I could not even reverse this.......WHO STOLE MY MONEY??