Mac or Windows? What version?
What exact version of LR? Have you updated LR 4.x to the most recent 4.y version?
Not visible where?
What do you see in the Library grid?
What do you see in the Folders panel?
What do you see in the "all photos" collection at the top?
Hoping for the best, does Finder/Explorer show your files still on the hard drive?
Show us a screen capture of the problem.