After upgrading a client's site from Drupal 6.12 to 6.13, the FAQ module stopped displaying the main FAQ page. I flushed the caches, tried visiting the site as an anonymous user, checked the error logs... nothing was working.
It turns out the trick is in the taxonomy module. Apparently on upgrade, sometimes the vocabulary you're using to categories FAQs gets disconnected from the FAQ content type.
To fix this, log into your site as an administrator and go to the Taxonomy page at admin/content/taxonomy/list. Find your FAQ vocabulary (mine is called "FAQ Topics") and look under the "Type" column. You'll probably notice that it's empty, even though it should be associated with the content type FAQ.
To fix this, click edit vocabulary, scroll down to Content types, and tick the checkbox next to FAQ. Then, click Save.
Now, visit your FAQ page. You should see a nice orderly list of questions!