One issue that can cause this is using Nikon software to transfer or view the raw images that is older than the camera. This is known to corrupt the image and make the unrecognizable to Adobe software. You can reverse this corruption using a utility found here:
A way to test if this is the issue would be to transfer new raw images from the camera or card to your computer without using Nikon software, but instead using the OS apps like Explorer or Finder.
Does Camera Raw come with Adobe Photoshop or does it need to be downloaded separately (Adobe - Adobe Camera Raw and DNG Converter : For Windows)? To check if it is file corruption or simply missing software, try to find test files on the net (dpreview Digital Photography Review: Digital Photography Review does provide RAW files for different cameras...).