I resign from my position at my previous job. After resignation I was unable to access my retirement . After endless calls they decided to issue me my retirement funds. Then months later I received a invoice in the mail stating that I was overpaid and I o

I resign from my position at my previous job. After an attempt to receive my retirement pension. I was made aware that I was overpaid for 4 months. They still issued my retirement pension and sent out a W-2 form. The woman who made me aware of this overpayment never dealt with in overpayment before and didn't know what to do I continue to contact her to find out what was going on and she didn't have any information to give me. So I continue to file my taxes and forgot all about it moved on to a different employer. Then six months later I received an invoice stating that I owe x amount of dollars. What should I do?