FNB took my money without any explanation

Unexplained debit order from my account. I’m very disappointed with FNB.recently, money was debited from my account without any explanation.i did not authorise this transaction.and when I contact FNB for assistance,I received no clear help, I did not get help at all.I expect transparency and proper feedback when it comes to my money.please investigate this matter urgently and explain why the debit occurred.i am extremely frustrated and disappointed with this FNB.i want this issue resolved and my money returned.