If you lost your IRS refund check (or failed to cash it in before the 1-year expiration date), call the IRS at 1-800-829-1954 to initiate a refund trace. The Bureau of Fiscal Service may need to review your claim before they can re-issue your IRS refund check, which can take up to 6 weeks to complete.
For lost state tax refund checks, contact your state tax agency.
Note: Generally, tax refunds for tax years 2014 and earlier can no longer be reissued, as the statute of limitations has expired. For tax year 2015 refunds, the deadline to replace a lost/damaged check is April 15, 2019 (October 15 if you filed a 2015 extension).