Hi, I am currently experiencing the same issue as you, and its driving me crazy. I know how you feel.
All my images in LR are basically dead. I can see them, but I cant work on them. Its as though LR5 is dead on my PC.
I use NEF too, some of them have asked me to change the file extension to a DNG. I have yet to try it, coz for the past 2hr, I have failed to 'Back up' my da** LR catalog.
I wish that you solve your issue soon.