>Suddenly, for no reason that I know of, .chm help files will not display. Even the VFP help file won't display in the VFP IDE, whether I press F1 or try to invoke the help file from the menu help item. Everything is on the local C: drive. Any help appreciated. TIA.

Did you install and then uninstall a VFP app which included help? Some installers also uninstall VFP help engine.