If you have an archived project folder - I find this
annoyingly necessary - copy the .hhc file from it. Close RoboHelp,
open the working project folder, and paste the .hhc file into it.
While you're there, delete the .cpd file. Open the .xpj file, and
RH launches and regenerates the .cpd file.
When you have a project that compiles correctly, copy the
entire project folder and make subsequent changes to the
copy.