Did you move from TP7 to TP9 ?
In that case you would need to convert the existing database to the new format.
Do you usually have Auto-creation of Thumbnails enabled ? (I don't like it, but some do.) With auto-creation on, TP will create thumbnails for all files in the folder that is currently displayed. If it is off you need to select the files you want thumbnails for and press F3.
-Michael