Sorry Martine, if you Deleted a Project, and indicated OK when the pop up that asks if you really want to delete this Project appears, MIT assumes you really want to DELETE and erases the Project.
Did you make an aia? Did you make a copy of the Project (Projects > Save project as )? If you did either of those, you can recover all or most of the Project. If you did not, the Project is gone. You have to reconstruct it. The good news is you made the app once and probably can reproduce the app or something similar and make it even better since you did in the deleted project.
Situations like this are why the Forum recommends making frequent backups of projects using either method mentioned above.
Regards,
Steve