What is the location and the type of medium of the files that you were expecting to see thumbnails of?
If you are using the right database, the thumbnails must still be in it. If they don't appear when you browse to the desired location, they should still show elsewhere in a grayed-out drive/path in the tree (but careful: If Options | Preferences | Thumbnails | 'Auto Remove Orphans' is on, they will be removed when you try to view them).