Hi Giuseppe. If you really can't find the source files the page below will tell you how you can get the project back again. However you should pay particular attention to what might be lost and how to prevent this.
Reverse engineering is a pain. It takes an immense amount of time to clean up all the files, recreate your table of contents, index, etcetra. Depending on your project size, reverse engineering will take days or even weeks.
I offer a script that can reverse engineer your output automatically in minutes: http://www.wvanweelden.eu/premium-content/reverse-engineer-robohelp-project-webhelp-incl-p ro-output
Hope that helps. Greet,