I have a similar problem. The thumbnails disappeared before I updated the site to WP 3.3.1.1
However when new images were uploaded, the new thumbnails appeared but the old ones or existing thumbnails did not.
I have upgraded to latest WP, the latest images that were put up are ok and show as thumbnails, but the previous images do not appear as thumbnails
Need some help on how to fix this, I have had the memory increased to 256 m but this has not helped.
