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.
Thank you for the reply. Prior to receiving your reply I had tried a bunch of different attempts and I finally got one to work - sort of. I don't remember exactly what steps I used but basically I found the catalog backup directory and opened one of the backup files. This allowed LR to start with that catalog. Unfortunately, the photos I had been working on (155 of them) when the problem occurred did not have the adjustments I had made. Additionally, LR still could not be started using the Desk Top short cut. It kept giving me the same "preview Cache error" msg. I figured this was because LR was still trying to start using the default catalog. I deleted the catalog that was in use when the problem occurred (I think it was named, "Lightroom 5 catalog.lr..). Anyway, LR appears to be working now. By the way, I discovered a directory named, "Corrupted catalog" and deleted the file (not the directory). Again, thanks for your answer. It will save me a lot of time if the problem occurs again.