This may happen if you get a different UID from the one you had before (this will remove you the ownership of those files). Try executing this on a terminal:

sudo chown -R $USER:$USER ~/.thumbnails

From 12.10 onward, thumbnais are also stored in ~/.cache/thumbnails, so you may repeat the same process for this location.

If none of those solves your issue, you can always remove them (they are automatically generated, and I think failed thumbs generations are cached):

sudo rm -rf ~/.thumbnails/* ~/.cache/thumbnails/*

Note: it's not the case of OP, but if you don't get previews to video files (ex: mkv, mp4) probably you are missing the video codec. This can be easily fixed by open the video with "Movie Player": it will propose a suitable codec.