What happens if you use the K: drive version of the card reader further down?
One thing that can mess up NEF files is copying/viewing them with any Nikon software that is a little older. This obsolete software changes something in the NEF files that makes them unrecognizable to Adobe software.
Another thing to try is instead of using File / Import, open LR to the Library module and then drag-and-drop the files or folder onto the central area. This should initiate an import operation on those specific files/folders.