Lost Source Code

No way. Re-construct it from scratch.

All you can get are screen shots from Appstore. But you should also keep your existing app on your device and make screen shots of anything you can. That will help you rebuild.

how can I get the source code back?

As others have said, this is unlikely to be possible )-:

What tools did you use to build the original app? Xcode? And in which language?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"