as far as i know, there is no method to retrive the actual sourcecode. i don't think darkbasic pro puts the sourcecode inside the exe and i could't find any dbpro decompiler
i found a similar thread (well the question there is the complete opposite and its from 2011):
https://forum.thegamecreators.com/thread/182372
Benjamin wrote: "If someone were willing to waste many months of their life writing a decompiler, it's technically possible. But who's going to do it? And why?"
so i guess your best chance is to write a new project from scratch. you can try to disassemble the exe to assemby code, but i don't know if that's gonna help you
a teacher of mine wrote: "data, without any backups, isn't important data"
good luck