Transfer a program from external back to mac hard drive

My husband did something really stupid! we got an external harddrive so i can store all my photos and back up the mac too. however, I upload my photos to aperture---well he move the program to the external and I believe it was deleted off the mac harddrive. It does not work properly anymore. How do I move it back without losing my clients photos that is currently being worked on.

Welcome to the Mac Forums.

Do you have your original install disk for Aperture? Just reinstall it on your internal hard drive. If any of the files were moved or missing they'll be replaced by the reinstall. After the reinstall is completed, you can move your photos back to the Aperture library. Just make sure you keep a backup copy of all your photos before moving them back.