Wrongly accused

I have recently beem threatenend by lawyers from house and home for a payment i have done and sent a proof of payment to them, they keep saying i havent, when i have sent proof twice that i have done it. Now my credit record has been affected because some idiot there isnt doing their job. This is completely unaccetpable, i have made payment and i am getting treated this way. I want this resolved and i want it resolved very soon.