|I would suggest reinstalling the browser, but AT&T no longer offers/supports it. They are stirring everyone to Internet Explorer 8 Optimized for Yahoo! That also means they won't be offering security updates.
Since AT&T Yahoo! browser is based on Internet Explorer, it likely stores your bookmarks in the C:\Documents and Settings\[YOUR USER NAME]\Favorites. You can try looking for your favorites in that folder. I would recommend using IE8 or Firefox as these versions should be more secure than your current browser.
If you want to continue using the older browser, but still want access to your favorites, you can create a shortcut to your favorites/bookmark folder on your desktop and access the folder from there. If going this route, to add future bookmarks, you should be able to right-click on a page and select "Bookmark this page."
Prior to installing one or the other new browsers, backup your favorite folders just to be safe (copy & paste files into a new folder). If AT&T Yahoo! browser doesn't store the bookmarks in C:\Documents and Settings\[YOUR USER NAME]\Favorites, you can try doing a file name search using one of the site's names through Windows search. From there you can locate the folder your bookmarks are stored.