I just went through a little hell. Upgraded from 2.0.1 to 2.2.1, only to see some tags not work. The navigation tags (next page, previous page) would simply not work. So, unable to find a solution (despite posting here several times), I downgraded to my previous version by uploading a backup. But then, I was told my db was "out of date" and had to be updated to the latest version. I just had to press a button, and it would do it automatically. Specifically, I have not yet uploaded a backup of the db, since I didn't want to lose all the posts written between my last backup (three days old) and when this happened.
So I just pressed the link that said "Update Database", and all the pages I had disappeared. No more "About Us", "Contact Us", etc. So, after looking around in the db, it seems WP thinks they are now regular posts. And their URL has changed to include the date.
What could have cause this?