Call the Internal Revenue Service at 1-800-829-1954 and request a "Taxpayer Statement Regarding Refund" form (Form 3911). Or if you prefer, visit the official Internal Revenue website and download and printout the form.
Fill out "Form 3911" completely and sign it. Make sure you go over the form to check and correct any errors.
Mail out the completed Form 3911 including your old refund check. Choose certified mail and return receipt when mailing out the form so you can have proof that you sent the form and get a copy of the recipient's signature.
Wait four to six weeks for your new refund check. If you haven't received your new check by that time, contact the IRS and follow up on it.