If you unzip the .capx, you will see that the main .caproj file is missing. All the images and sounds are still in their respective folders, but without the .caproj file then the program cannot launch.
Hopefully you have automatic backups and autosave setup. If you look in the same folder as your project, you should be able to see if these exist and you can simply rename them as "project.capx" and try to launch.
More information on autosave and backups can be found HERE.