I've been following this thread. The same thing happened to me more or less out of the blue after a long period (many months) of satisfactory operation. Tried all sorts of things including a virus scan to get the thumbnails to display in the library while importing. Everything else, i.e. develop and print worked fine so the images did get imported, just not displayed in library. Finally, I uninstalled LR5.6, reinstalled it from the original disk, updated it to 5.6 and everything has been fine since (perhaps 10 imports so far). Imports were raw images (all that I use), also from a D7000, Computer is a Dell 620 that has been in use for something like 2 years. OS is Windows 7, Home Edition with Service Pack 1.

I've been following this thread. The same thing happened to me more or less out of the blue after a long period (many months) of satisfactory operation. Tried all sorts of things including a virus scan to get the thumbnails to display in the library while importing. Everything else, i.e. develop and print worked fine so the images did get imported, just not displayed in library. Finally, I uninstalled LR5.6, reinstalled it from the original disk, updated it to 5.6 and everything has been fine since (perhaps 10 imports so far). Imports were raw images (all that I use), also from a D7000, Computer is a Dell 620 that has been in use for something like 2 years. OS is Windows 7, Home Edition with Service Pack 1.

Back up your Catalogue
Close Lightroom
Navigate to your previews folder, it will usually be in Pictures\Lightroom and will be called
"Lightroom 5 Catalog Previews.lrdata"
Change the name to "Lightroom 5 Catalog Previews.lrdata.old"

Now open Lightroom and goto the Library
Menus > Library > Previews > Build Standard-Size Previews

The previews should rebuild )this could take a while depending on how many images you have)

If all is well, Back up your catalogue

The reason for renaming the old version was to preserve things in case anything goes wrong. The old file can be deleted if everything works out OK.