Backup the project first, then try deleting the CPD file and reopening it.
If that fails, take a look at http://www.grainge.org/pages/authoring/opening/opening_projects.htm for other methods of opening a project.
See www.grainge.org for RoboHelp and Authoring information
Thanks Peter. I contacted Adobe and they had me remove both the .cpd file and the .xpj file and then reopen using the .hpp file... and that worked!
OK, that makes sense. I had tried deleting just the .cpd file before contacting Adobe and that didn't work.
Fortunately, it seems to be ok now.... fingers crossed.