Can I recover the Xcode project of an app I uploaded into the AppStore? I lost my previous hard drive (where it was created and stored) and now I would like to keep improving my app but the only link I have to the project is iTunes Connect. Is there any way to do it?
No. What you submit to iTunesConnect is just the "artifact". None of the original code files or project files go up. You can get art assets (like images or sounds) out of the uploaded ipa. But as far as building the project, you'll have to start over.
As stated in comments, this is why you use version control and backups.