So your rss feed stopped making new threads 3 days ago? What did you do three days ago on your site? Did you add any new modifications? Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
The RSS feed for your site will not show new posts, only new threads. And, if will only show new threads in the forums that are visible to guests/unregistered users. (I see several threads on that feed.)