Someone filed a claim against me with a collection agency and I feel I do not owe the debt.

I worked as an independent distributor for a company. On one occasion I signed a promissory note with the headquarters for merchandise. I have had several notes with them and have always paid them off. I never receive a bill but my price on merchandise would be $100 or $200 higher each time Id order Id pay the extra charge towards note. My regional sales director was upset I looked at another company that sells 2 very different ite and froze my account. I could no longer order any merchandise. I asked for his reason in writing and he would never provide it. He now has a collection agency after me for the remaining note, that was never between him and I, and due to him unfairly freezing my account I could not stay in business nor purchase merchandise in turn paying on my note. What can I do