Mac laptop, OS El Capitan, LR 4. Approx. half of my 7000 pics have gone "missing". The thumb nail pictures are still there.
Missing? DO you mean there is a question mark icon on the photos? Or do you mean something else?
If it is a question mark, then you'd have to reconnect the photos using these instructions: Adobe Lightroom - Find moved or missing files and folders
Retrieving data ...