Sounds like a corrupted preview cache. Close LR and find the catalog on the hard drive. Rename the lrdata folder (Windows) / file (Mac) - don't touch the lrcat catalog file. Then open LR and see if the previews reappear. If they do, select all in Grid view tonight when you go to bed and go to Library menu > Previews > Render Standard Sized Previews - it take a long time so it's worth running overnight. If everything looks good in the morning, you can delete the lrdata folder (Windows) / file (Mac) that you renamed earlier.
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