[The only open topic I could find involved reinstalling v.3 and Twenty Ten]
Site has been functioning since April, 2010. Last update to theme (style.css) uploaded to server on 11/22/2010. Poking around the site, getting ready to design a new content type, I clicked on a link and the page showed up with the Default 1.6 theme. On the Appearance screen, only Default and Classic are listed. Logging onto the server (Bluehost, manual install of WP 2.9.2, untouched since), I confirmed that my theme (FPC) is in the themes folder where it's always been. It simply doesn't show up as an available theme. No themes are listed as broken. I uploaded a fresh copy of the the theme, still no luck. This copy works on a separate test site that is also running a manual install of 2.9.2.
Any help would be appreciated. Thanks.