Using LR 5.7.1 on a MAC. I was able to import NEF files with ease and now I am unable to import or view any prior NEF photos. I've reset all settings, uninstalled and reinstalled with no luck
Could you please confirm the camera make and model ?
Do you receive any sort of error while importing?
No errors, the button is simply is greyed out. When I drill down to the folders I don't see any photos available. When I go the desktop via Finder they are all there. I'm primarily using a using Nikon D300. I'm also experiencing the issue with photos from a Nikon D50 or any point a shoot camera.
Go to File menu > choose Import Photos and Videos
If that does not work, you should reset Lightroom Preferences.
Refer this article for steps : Resetting (or 'Trashing') the Lightroom Preferences file - Lightroom Forums