I sold my house 12/2018 but I did not receive a 1099-S. IRS sent me a letter stating realtor sent them a 1099s, now I owe the IRS the gains, what do I do?

I called the realtor and the lawyer and both stated they do not have a 1099-S for the sale of the home. I can't send an amended return because I don't have that form, what should I do?