Are you sure this isn't a scam letter? Correspondence regarding your tax return is usually has a letterhead indicating that it is from the IRS and has an IRS logo with no mention of the Department of the Treasury rather than indicating that it is from the "Department of the Treasury" and having a Department of the Treasury logo (although the IRS is under the Department of the Treasury).
To be sure that you are contacting the IRS you can contact them via the methods listed on the IRS website. Until you have verified that the letter or contact information on the letter is legitimate, do not contact using any of the methods listed in the letter.
https://www.irs.gov/
