Don't give up hope. I had something important show up after missing for two months.
In any event, file a search with the USPS Mail Recovery Center; perhaps the passport and envelope got separated:
https://www.usps.com/help/missing-mail.htm
It looks like U.S. passports found loose in the mail are automatically returned to the State Department, but whether that includes cancelled passports I don't know. In any event, you might want to contact the State Department as well. Do a Google search for an office there called CLASP.
Don't give up hope, indeed. The State Department admitted over the phone my old passport was lost and I should get new visas, but it showed up today in the mail months late.