You can do that with this plugin if you have CaRP Evolution:
http://www.geckotribe.com/rss/carp/CaRP-WP/
You can get CaRP at http://www.geckotribe.com/rss/carp/ or http://www.geckotribe.com/rss/rss-to-dollars/
Possible reasons for such a thing happening (off the top of my head):
1) If you’re using mySQL caching with CaRP and changed your mySQL password, you’ll need to update carpconf.php.
2) If you’re not using mySQL caching, but somehow changed the access permissions on CaRP’s cache folders, you may need to delete the cache folders and rerun the CaRP installation script.
BTW, there’s a new WordPress plugin for displaying RSS feeds inside WordPress posts at http://www.geckotribe.com/rss/carp/CaRP-WP/
