What camera took these RAW photos?
What version of Lightroom are you using?
Have the files been opened in any other software such as Nikon NX which may have removed profiles to save space?
Try shooting a few new raw files, then copying directly from the camera/card reader.
Or copy files into a folder on your hard drive and use Import >> Add
LR will build its own previews.
Canon 6D and I'm using LR 4.
Or from the LR menu simply click:
Help >> Updates