Over payment, they want money back.

You are not alone.  An overpayment happens every day to many people, for many reasons.  Because you didn't list why they overpaid you, I won't speculate, but I can offer some suggestions to take now that it has happened. Within TEN (10) days of the notice, appeal AND ask for a waiver  so that your monthly check will not stop.  Note that if you are unsuccessful, any money they paid you during the appeal will be part of what they are collecting. If ten days has gone by, appeal.  You have sixty (60) days from the date of the letter. Ask for a waiver of the overpayment if you don't have the money.  You will need to provide financial information to SSA so be sure that it is accurate. You also can choose to pay back the money, and if your benefits are continuing, you can negotiate an amount to be withheld each month.  I would suggest that it be a small amount. Good luck.