You entered information from those forms if you e-filed, but not the documents themselves. Did you save your copies of your W-2's and 1099's? You should always save them for your own records. The iRS wants to see if they match what you entered on your tax return. They are looking to see if you correctly entered the amount withheld for federal taxes that you entered on line 64. Make xerox copies for yourself if you need to, and send the documents to the IRS. If you threw them away or lost them, ask your employers for new copies of the W-2's; you do not say what kind of 1099's you had but you could most likely get copies from the financial institutions that issued them.
Send the photocopies to the IRS and keep the original documents for yourself. Never send original documents to the IRS (except for copy B of a W-2 that you attach to a tax return that you file by mail).
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
It is also possible that your employer failed to send the IRS copies of the W-2 forms, or remit the actual tax dollars that were withheld.