@Bet6
Read the mail request from the IRS carefully again...Be sure they are not also asking for any other forms that show Federal withholding too. Like W-2 or 1099-R forms. So that you send them everything they want. (It did come as a Post Office mailing on paper correct? NOT an e-mail request. E-mail requests are likely scams)
Also look at your actual printed form 1040. Check the lines 25 a,b,c,d to ensure the withholding totals you entered actually matches what's on the various tax forms you entered.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
