Debt collector?

Tara • Have two beautiful children
Guys, I have never had this happen to me ever so I have no clue what to do. I have a letter from a debt collector saying I owe a bill that I paid back in November. I pay everything through my bank online and have never had a problem. I went back and found that they had received the check and cashed it. I have tried a million times calling this collection place that says 24 hour but no one will answer my call. I have tried everything to get a hold of them and even called the place that the bill is from. Nobody seems to give me any answers. What the hell do I do? I don't want this going against me when I paid it three months ago!