Call the IRS at (800) 829-1954. If your refund check has not been cashed, we can normally provide a replacement within six to eight weeks. You may need to complete a Form 3911 (PDF) Taxpayer Statement Regarding Refund, to initiate a claim. If your refund check has been cashed the Financial Management Service (FMS) will provide a claim package which includes a copy of the check. FMS will review the claim and the signature on the cancelled check before determining whether another refund can be issued.

Source: www.irs.gov