Don't panic, it's just the preview cache. Find your catalog on the hard drive - usually in the Pictures folder called Lightroom 5 Catalog.lrcat by default. You'll see a *Previews.lrdata folder there. Rename it and restart LR.
LR will have to rebuild previews, but then everything should be ok. You can let LR rebuild the previews as you need them, or you can select them in Grid view and go to Library menu > Previews > Render Standard Sized previews.
Once you're happy that everything's rebuilt, you can delete the one you previously renamed.
Thanks MB. I will give it a try now.
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I'm thinking there is probably an issue with the file where LR stores it's previews. On my Mac, the files are located in the same directory as the LR catalog and they have the extension *.LRDATA (I'm not sure where they are on a windows machine).
You might try renaming or moving these files so that LR can just recreate them the next time it opens. If that works, you can just have LR recreate all of your previews to get you back to where you were before you were having problems.
In my hard drive I found the following directory: Libraries > Pictures > Lightroom 3 Catalog Previews Irdata
I'm on LR 5, but I changed the name of this folder and restarted LR5 and got the same result as before. I don't see a LR 5 Catalog.Ircat, but maybe it's hidden somewhere else?
I did a search to find a *.LRDATA directory and did not see one pop up. I'm at a loss as to where to find this directory. Any ideas on how to better conduct a search?
Got it Brian. Between you and a couple of other folks you got me close enough to figure it out. I store my backups on an external hard drive and that's where I found the LRDATA file.
I was desperate and it was just rename the folder name! Thank you, you saved my life!