Lightroom encountered an error when reading from its preview cache and needs to quit. Lightroom will attempt to fix this problem the next time it launches.
It hasn't fixed the problem. How can I fix it?
With Windows Explorer or Mac Finder, go to the folder that contains your LR catalog, typically inside Pictures or My Pictures.
There will be a folder, named [your catalog name] Previews.lrdata
Delete this folder - do not delete anything else.
Lightroom should now start normally, and will rebuild the previews.
Assuming Lightroom is in the normal folder location, go into your computer's Pictures folder, then subfolder Lightroom, and look for a file called something like Lightroom Catalog Previews.lrdata. Delete (or just rename) this file, nothing else, and restart Lightroom.
Your solution worked. Thank you for your prompt reply!