You can NOT get the source back from the compiled version. You may get the java-source with a decompiler. But that´s not the B4J Source you build the app with.
Fact you´ve learned today: Do make Backups of your Sources!
Perhaps you can recover your program source code (partially) from backups in the backup directory, otherwise it is an instructive seminar that you should make a backup regularly to avoid having to start all over again.
Perhaps you can recover your program source code (partially) from backups in the backup directory, otherwise it is an instructive seminar that you should make a backup regularly to avoid having to start all over again.