Since nobody has offered anything so far, I found this after doing a topic search. I don't know if it applies to you, but you can try it and take care to follow the advice re the file types! Hope it helps.
Do the thumbnails (or rather, the places where the thumbnails should be) have icons on them (a question mark in the upper right corner) indicating that the photo is missing?
If so, then the steps below won't work. If not, then try throwing out your Lightroom previews file by following these steps.
1. Quit Lightroom.
2. Go into the Pictures folder on your hard drive.
3. Go into the Lightroom folder which is in the Pictures folder.
4. In there you'll see a file called "Lightroom 3 Catalog Previews.lrdata". Put it in the trash. (Don't empty the trash yet, that way you can always drag this file back out of the trash if something goes wrong.
Important note 1: Do NOT throw out the file called "Lightroom 3 Catalog.lrcat". This is your Lightroom catalog file, and it contains all the info about your pictures. You definitely don't want to throw it out.
Important note 2: The locations and file names that I mention above are the defaults. It is possible that you've named things differently, or put them in different places, in which case you'll need to find things wherever you put them. Regardless, the Lightroom preview file ends with the extension ".lrdata".
After throwing out the preview file, relaunch Lightroom. Initially, all the photos (even the new ones) will appear as blank, gray thumbnails. But then the thumbnails will all regenerate, including (with luck) thumbnails for the photos which are currently missing thumbnails.
Also
If there is a question mark on the thumbnails, then the steps I suggest above won't help. The question mark means that Lightroom cannot find those files on disc. This means that the files were moved on disc, or deleted (outside of Lightroom), and now Lightroom doesn't know where they are.
You can click on the question mark and Lightroom will ask you if you want to locate the missing file. You can then tell Lightroom where the file is now located. Lightroom will automatically find other missing files that are nearby.
