Could someone give me some advice on a debt collection question?

My former husband passed away in 2006. At that time I notified the credit card companies of his death. I had a card in my name and I have recently paid that one off, but another card that had dismissed the debt due to his death apparently got wind of that and has started to dunn me again. The company that I am talking about now sent me a letter stating that I was no longer going to be bothered with that bill. Now that I have remarried and I no longer have that letter that I got in 2007, I have been getting calls from them trying to collect that debt and just last summer a debt that was $11,000 in 2006 is now over $17,000 due to interest. What can I do about this. I kept the letter from them for over two years, I didn't think that I would start getting dunns from them again in two more years. I did not throw this letter away, but I have moved 4 times since I recieved it and I cannot find it. The last time I was confronted with this, I tried to explain that I had gotten this letter and the woman said that she did not believe me and wanted to know why it was sent to me. I told her that I had no idea why I recieved it, but I could not locate it now. What will happen if I can't find it?