After you follow Derek's posting to turn the Thumbnails back on.
Then check to make sure when you run Disk Clean Up that you don't delete them every time you run it. Disk Cleanup - Open and Use
As in my Snip below make sure the Thumbnails box is unchecked.
If checked it will delete the Thumbnails.
Then you start over to rebuild the cache which can take a few days.
I know I deleted mine by accident.