This is actually a huge problem, and I'm not doing anything with my phone until I've recovered the files.

Basically, I have a folder on my Internal Storage on which I work, and a similar folder on SD Card where I copy all the files from time to time because I run out of space in Internal Storage (then I copy the files from SD card to my computer).

Today I wanted to do it.
I opened the parent folder in Internal Storage on left screen, and parent folder in SD Card in the right screen.
In the left screen, I selected the folder I wanted to copy, pressed Copy, went to the right screen, and pressed Paste, confirming that I want to replace the existing files.
This went super fast, which shouldn't have happened considering the size of the files. And I realized I had opened the Internal Storage folder in both screens. So basically I copied a folder over itself. Which shouldn't be a problem, normally it should just leave the files the way they are... Except it didn't. ALL MY FILES in this folder have turned into 0KB files. They can not be opened anymore, they're completely empty.
I've just lost 1.25GB of work.