What you see are probably JPEG thumbnails embedded in the NEF file. They display correctly if the first kilobytes of the file are readable, but that does not mean that the rest of the file is not corrupted.
Some points to check:
- the size of the files reported by the camera (if the camera can display it) and is it compatible with the size of your NEF file when the camera is working?
- the size of the files in the file explorer of your computer, and if that seems consistent with a NEF size.
- if the files are large enough to be NEF files, zip them (ZIP from any other compression utility, such as WinRar). Compare the size of the ZIP file with the size of the files. If you get a compression greater than 5%, the file is probably damaged (the NEF files are probably not well compressed, while the damaged files often contain large blocks of data replaced by zeros or zeros and these areas compress themselves very well). You can zip a few good NEFs to get a better estimate of the compression ratio of the NEFs (2% for my Canon CR2 …).
The tests above will tell you only if it is possible to hope to recover some files. As for Why the files are damaged, as @flolilo says, too many variables … If the cards behave normally in another camera, it can work from the camera.