When a project is saved, the VPJ file contains pointers to the source files. Apparently something changed, as VP can no longer find them.
You might COPY the VPJ to the same folder as the source files. Or copy the files to the VPJ folder. That succeeded in a simple case here, where a file had been deleted.
Once resolved, SAVE PROJECT FILE AS, giving it a unique # or name, so as to not overwrite a prior save that might one day be needed.