You will need to contact the IRS or state taxing authority (whoever sent the check to you).
Telephone Assistance for Individuals:
Toll-Free, 1-800-829-1040 Expect long wait times.
Alternatively, this link has the local IRS offices. After going to the link, you can click on the map for your location. Contact Your Local IRS Office
How do I contact my state's Department of Revenue?