What can i do if i forgot to turn in the RELEASE OF LIABILITY to the DMV after selling a Vehicle?

I sold a crappy old Astro Van i had for 500 bucks. I got a bill of sale and release of liability signed and complete but i forgot to turn it in. 2 months later i get a red light ticket in the mail for that vehicle with the guy i sold it to driving it. So i looked and looked for the bill of sale and release papers but i cannot find them anywhere. What can i do.?