There are a few clues that your preview cache may be corrupted. The most obvious is Lightroom showing an error message telling you that it’s corrupted (“Lightroom encountered an error when reading its preview cache and needs to quit.”), or that it tried to repair it unsuccessfully.
Alternatively you might find that Lightroom displays the wrong preview for some of your photos, some previews or thumbnails disappear when the photo is deselected, or the photo only shows in the Develop module.
In Lightroom CC/6, Lightroom tries to fix the preview cache automatically when it restarts. If this doesn’t work, or you’re using an earlier Lightroom version, you can fix it by manually rebuilding the preview cache:
- Find your catalog on the hard drive. If you can open Lightroom, go to Edit menu (Windows) / Lightroom menu (Mac) > Catalog Settings > General tab and press the Show button to open an Explorer (Windows) / Finder (Mac) window, then quit Lightroom. If you can’t open Lightroom, and you don’t know where to find your catalog, you’ll need to search for *.LRCAT les.
- Using Explorer (Windows) / Finder (Mac), move or rename the previews folder (*Previews.lrdata) and restart Lightroom.
- Lightroom can now rebuild the previews from the original files. Select all the photos and go to Library menu > Previews > Build Standard-Sized Previews and leave it to work, perhaps overnight as it takes a long time. If any of the photos are offline (e.g. stored on disconnected external hard drives), these previews will remain blank until you reconnect the drive.
- Once you’re happy they’ve all rebuilt, you can delete the previous previews folder (*Previews.lrdata) if you haven’t already done so. I don’t usually recommend deleting the old previews before building new ones, just in case there are any missing original files, and the previews are the only copy you have left. If you’re comfortable that you still have all of your original files available, then you can delete the old previews before building the new ones.