CSA told me everything was fine, then changed their minds

I had someone from the csa come out to see me as my ex partner had contacted them I give all the info he need plus he told my pay slips I dint here anything back after 4 weeks so phoned them was told all is fine didn’t here anything so phone them again they said if I have to pay they will contact me didn’t here anything for 5 years they just took a attachment of earning order saying I owe £15000 they are saying they cant find anything from 5 years ago and when i complained the letter i got back basically said i don’t know why your complaining is there anything i can do?