credit card payoff

I paid a credit card off a year ago and it reflected as paid on Transunion. (was in collections, so it showed as paid charge off) Received a new card from the same company this year. When I pulled credit bureaus it is showing on experian and equifaax as unpaid collection. i have since lost the original payment receipt and the company is no help. How can I fix this? Collection company is seeking payment again.