Two possibilities: Corrupt Preview Cache or a bad Monitor Profile
1. Preview Cache is located in the same directory as your Lightroom catalog and has the name "yourcatalogname Previews.lrdata" close Lightroom. Rename this and restart Lightroom. Do the thumbnails start appearing?
If that doesn't work, reverse the name change you did in step one and then try this:
2. Set your Display profile in the Control Panel to sRGB. Do the thumbnails come back?