I am having the very same problem and have tried fix and re download to no avail.
You need to delete the preview cache and let Lightroom rebuild it. The preview cache file name extension is LRDATA with the same file name as your catalog. Preview cache locations are shown here
When starting Lirghtroom 5 I get the following error message each time: "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 never launches.
I just posted a HELP on this very issue, and saw this after the fact. My problem now is that I thought I needed to uninstall it and reinstall it. Where do I go on CC to reinstall my lightroom? Anyone help please, I am editing a session right now, and I am very stuck.