Sorry to say this, but your file is most likely gone. Why do I think so? Because the smallest KRA-File I have has already 30kb in size, and that is a tiny test picture with only two layers and nearly no content.
It seems you already did the following, 
but I’ll write it anyway, maybe it will help others who stumble over this topic!
If you are trying to recover a corrupted file, you must first make a copy of it with your file manager, anything else has to be considered a tendency towards digital suicide, because it could shred your last straw. So if you try to rescue the file and that try should damage it, you have that backup from which you can make the next copy for the next try, but never use your last straw until you decide the patient has passed away.
Furthermore, you really need to read up what for autosaves and backups are meant:
The biggest problem with these functions are a not so lucky chosen name for crash-recovery files and users who don’t read a manual, because “assuming” should be sufficient (shouldn’t it?), combined it leads to fatal losses.
But since you need to prepare your process of file handling for the future, I want to point you to this post, where I describe possible ways to never lose that much work again, make your process of file handling with Krita a lot safer, including safety nets (from the second paragraph on):
This may be helpful too, but it is very similar:
Maybe it will help you.
Michelist