Do you know what to do?
Yes - delete your-catalog Previews.lrdata folder in with your-catalog.lrcat file.
Delete the whole .lrdata folder, and nothing else (e.g. do NOT delete your .lrcat file) - if all goes well, it should start right up after-wards .
Consider doing Library (menu) -> Previews -> Build Standard-Sized Previews to re-build lost previews, or just let them be rebuilt "naturally".