It sounds like Drupal suddenly doesn't know where your files folder is. Check the File system settings page and make sure it matches. Also check the settings on the image field in your content types and make sure that the directory it points to is in the file system.

Back when image support was first introduced, I had made the mistake of trying to make an ubercart_images folder for product images. It never really worked very well, and I finally got rid of that code recently. I think maybe that fix may have broken your site.

To answer your question: CCK creates the content_field_image_cache table which maps node revisions to file ids. The files table maps file ids to file paths. Imagecache takes those paths and copies them to each preset folder in its own directory structure.