Terrible Service From Virgin Active!!

Almost a year after cancelling my contract with Virgin Active, i get a message Vvm (debt collectors) saying i owe virgin active R 4672.91, and that an immediate payment of R934.58 has to be made within 48 hours??? I tried calling Vvm, no answer. This is terrible service, and if this affects my credit rating in any way, i will take legal steps!! How do i sort this out??