Yeah, looks messed up, I can't spend the time to figure out why for ya right now.
It might comfort you, just to try this: switch back to Default theme, and "see" your blog working, just with the wrong theme. For me that always helps me know the posts are still all there, which looks like yours are.
Also write to the Mandingo theme author & see if they are planning to update it to make it compatible with WP 2.6.
In the longrun you'll probably want to upgrade.... to 2.7, 3.0, etc.... so you don't want to stick on a theme that stays outdated.
One more thing, again, i really think you might need a backup OF THE DATABASE to downgrade. Forget the regular files, it's the database SQL file that you might need (not sure but you might). Have you got it?