How can I restore my image files?

I bought a new iPhone last year and I wanted to keep the pictures from my old iPhone. So I bought an external hard drive (not just for the pictures, but generally for more storage space). I don't know exactly how I dragged the pictures from my iPhone onto my Asus laptop, but afterwards I dragged the pictures from my laptop onto the external hard drive. On my laptop I noticed that I could no longer open some image files and video files, but I could on my iPhone. I hadn't thought of anything, but now I really noticed it. I then reset the iPhone to the factory settings and then sold it.

Is there any way to restore the image & video files because they are mostly vacation pictures and it would be a real shame if all of this was gone now.

So you just can't get it open?

Yes, so I don't know if I just can't open it or the file is broken.

But is it there?

The files themselves are there