This is the Photoshop forum however.
Close Lightroom and go to your Lightroom catalog folder.
The default location is usually in Pictures.
Delete the file in the catalog folder which ends Previews.lrdata
Do not touch the catalog itself which ends.lrcat
Then try re-launching Lightroom.
That didn't seem to work. Thanks though
It's a whole folder, and has the same name as the catalog you are using except for the extension. Did you delete the entire folder?
You can try deleting the preferences. That's something that seems to clear up a lot of little weird things.
No. Deleting preference files might be a cure all - not in this case though.
What this "encountered an error reading from its preview cache" message really means is that there is a problem in the ...previews.lrdata folder for this catalogue.
So to resolve it, in Finder/Explorer go to the folder containing your catalogue. Next to "Lightroom 5 Catalog" will be a folder called "Lightroom 5 Catalog Previews.lrdata". Delete this Previews.lrdata, restart LR, and things will be OK.
If this doesn't work, you've deleted previews.lrdata folder for another catalogue. That's no great harm.
The catalog was in 2 places and I deleted, problem solved. Thank you so much.