Taxpayer’s check returned undelivered by the Postal Service and the taxpayer does not meet Internet Refund Fact of Filing (IRFOF) eligibility. For example, an account may have an additional liability, or a freeze code other than S-. Analyze account and take appropriate action.

If you check Where’s My Refund (WMR) tool and see you have a reference code, you should identify the code and see if any action is required.
In many cases, no action is required on your part. If there is an issue, the IRS will typically send you a letter to your mailing address on file within 90 days stating what the issue was, and any additional information required.