First of all it's very important to understand the bibisco Projects directory structure.
To restore the work, if the project directory is present on the computer:
1) Verify in the Settings -> Directory of the projects that the directory is correct: it has to be the projects directory (the directory indicated with the purple arrow in Projects directory structure page).
2) If the pointing is correct and the projects still do not appear in the list of projects, import them individually as archive, selecting them from the backup directory (the directory indicated with the red arrow in Projects directory structure page).
If the projects directory is not present in the computer and you have a backup archive file (.bibisco2) you can import it as indicated in the previous point 2).
Ok, but why did it happen? Bearing in mind that the bibisco never deletes the project directory, the most common causes are:
The bibisco projects directory has been moved.
The bibisco projects directory has been renamed.
A new version of the bibisco has been downloaded to the computer and the bibisco projects directory has not been configured correctly (take a look at: How to update bibisco).