What to do if you have lost your IRS tax refund check?
I would urge you to call the IRS at (800) 829-1954. If your refund check has not been cashed, they can normally provide a replacement within six to eight weeks.
Per the IRS, "the Taxpayer would 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."