Checks sometimes get lost, no matter how careful you might be. As with all checks, tax refunds issued by the Internal Revenue Service or a state have expiration dates. However, as long as it's clear you never cashed a refund check in your name, it's possible to get a replacement.
Give it a Year
According to the IRS, a tax refund check is good for one full year from the date on which it was issued. It expires on the last day of the 12th month from the month you get the check. Once that year-long period has elapsed, banks consider the check void.
Requesting a Replacement Check
Expired checks issued by the U.S. Department of the Treasury can be replaced so long as the original was never cashed. The contact number for the IRS, which issues federal tax return checks, is 1-800-829-1040. They will send you a claim form you must fill out before a replacement check can be issued.