CSA said I owed nothing and 20 years later I get hit with massive bill

The CSA (or whoever they are now), have an attachment of earnings to my employer of £115 per week. They tell me I owe an extremely large amount of money to them. The child in question is 22 years old and I have never had sight of her. The last correspondence I had with the CSA was back in the late 90s whereby they wrote to me and said I owed nothing. How can I now owe this large amount? I have no records of what has been paid out or why as I know the mother in question has always worked and so does her partner who resides at the same address. My partner of 17 years has recently suffered a brain haemorrhage and may not be able to work again. I therefore need to support both her and myself but cannot do with such a weekly amount being taken directly from my wages. Where is the fairness in this so called law?