What happens when you try to edit your NEF files? Did you happen to use Nikon Transfer to download the NEF files from your camera?
Yes, I did. In the past I had no problem in doing that.
I don't know what may have changed that now makes your NEF files unreadable. However, some versions of Nikon Transfer cause the NEF files to not be readable by Adobe software. If you still have the images on the camera card, try downloading them directly without using Nikon Transfer. If you no longer have access to the images on the card, you can use this utility to "fix" the NEF files. I have never used it. I have never needed to.
Personally speaking, I find no reason to use Nikon Transfer to download images from the camera. The Windows and Mac operating systems have utilities to do this. And Photoshop Elements enables you to download. As far as I'm concerned, there really is no reason to use other software to download. Nikon Transfer doesn't do anything to your images that you would benefit from by simply downloading with it.