No thumbnails in Media Library

I had to reinstall WordPress because a hacker. I copied all the media files to the original path (wp-content/uploads) and restored the database. Everything looks ok. However when you go to the Media Library you can see the list of all medias (mainly images) but the thumbnail doesn’t show up. Also, if you upload a new images everything is peachy.
I deactivate all plugins and activate the Twenty Ten theme but problem persists. I also checked for the thumbnails in the uploads directory an they are there with the dimensions specified in the media’s settings.
Any idea?

Thanks,
Arty