It sounds like something in your preview cache was corrupted. No worries - you can delete the preview cache and Lightroom will rebuild it, although it may take some time, depending on the size of your catalog.
To delete the preview cache, do this:
1. Quit Lightroom.
2. In your Operating System, locate your Lightroom catalog. Typically catalogs are in /Pictures/Lightroom on Mac and My Pictures\Lightroom on Windows.
3. Find the folder whose name ends with "Previews.lrdata" and delete it. (note, you may also have a folder whose name ends with "Smart Previews.lrdata." You shouldn't need to delete that one.)
Once the preview folder is deleted, restart Lightroom. You may see gray thumbnails in the grid for a few moments as Lightroom starts rebuilding your preview cache.
Let us know if that helps.