We opened a project and the html files and TOC were gone.

First, thank you for your help. We have RoboHelp 11 project that was working fine. Then we opened, and the files and TOC did not display.  They were somehow removed from the project. We turned on the "Clear project cache (.cpd file) before opening any project." But that did not fix the problem.  Does anyone have any idea what a happened?  The project was working fine, then the files and TOC vanished from the project upon opening.  Many thanks!!!!!!!!!!!!!!!