The D750 has been supported since Lightroom 5.7. The last update was 5.7.1. If you have that update, and can no longer previews your NEF files then I'm not sure what might have happened. Do they import? Are there any error messages? Are you using Nikon Transfer to download the images from the camera? What exact version number of Lightroom 5 do you have installed?
Added later: if in fact you really only have Lightroom 5.0 then that is the reason you won't be able to work with the NEF files from that camera model. You need the last update that was made available for Lightroom 5.