line 64 is federal withholding. if you filed by mail all forms w-2 and any 1099's with federal withholding should have been mailed with the return. if you e-filed, it is possible you enter an incorrect number or no number when you should have. there is also the possibility that you had multiple employers and that the Social Security withheld by them exceeded the maximum. line 64 would reflect this excess/ in any case send copies, never originals, to the IRS at the address stated in the letter
