FNB are out to ruin lives

I had an account with FNB and i closed the account, all of a sudden i have been receiving calls from a collection agency. Saying i owe FNB R8k+ for an account i closed about 5 years ago. I dont have access to all the correspondence as i was retrenched. R8k for an account that has not been used for 5 years plus. NO correspondence or phonecalls from the bank, only the collection agency all of a sudden. FNB are really making my life difficult. How can we not help you