File Closed & Overpayment

Got a letter that my file was closed and I have quite a shockingly large overpayment that I have to pay back. The letter said it was because I received OSAP, but I told them I was receiving and kept getting my payments. I'm trying to get in touch with my caseworker - classic no response. IDK what to do? I have no way to pay this back and there's threats in the letter that they will notify the CRA.