Question on a serious issue with an over payment check I recieved!!!!?

Ok so a brief run down of my issue. I started to get ssd in 2001 after about 3 years I tried to go back to work but still couldnt hold a job and finally refiled for it 9 months ago. They started to give me checks til they decided on an answer after 6 months they approved me and then I recieved a 12 month back pay. about a month and a half ago they sent a check in a rather large sum lets just say it was in the 20,000 range. I called first thing monday morning and asked what the check was for and if I should cash it and the guy said it was issued to me it was mine and it was ok to cash. now thgis past saturday they sent a letter saying the check was issued on accident and that I need to return the full amount with-in 30 days. Now I have been scraping bottom for a long time and I have spent most of the money on a newer and more dependable vehicle. pay off past due bills. WHAT DO I DO? Can I go to jail? how will I be able to pay this back