I had a defaulted student loan, but paid it off now I'm getting a call that says I owe more. What can I do?

I had defaulted on my student loan and they were harrassing me. My husband had recently passed away. I had no income, no insurance and had to have surgery. The District Attorneys office called and I had to meet with them. I managed to get a loan with the help of a friend and paid the student loan off. Now 2 years later, I receive a call that says I still owe a student loan. I don't believe this is true but I never received a letter stating the loan was paid. I don't know how to prove that the loan was paid.