This is just a guess, mind you, but it’s still worth a shot.
First, turn on debug mode and see if there’s any glaring errors when trying to view your content. If not (and I’m thinking there won’t be, but I would feel quite silly if there were errors pointing to the issue), then roll everything back to 2.8 (database and files)and manually upgrade to 3.0.x. If your content is still there once that’s done, back up your database again then upgrade to 3.3.x.
Edit: At this point, it’s either a theme issue or a plugin issue. The function register_sidebar_widget has been replaced with wp_register_sidebar_widget, and register_widget_control with wp_register_widget_control. First things first, switch to the default theme (Twenty Eleven). If your content shows up, we’ve narrowed it down to the theme, and you can just find/replace the former function with the latter.
If the theme’s not the problem, then it’s a plugin generating a specific widget. Remove all your widgets and re-add them one by one. Whichever plugin generates the widget causing the failure is the offending plugin, and can be updated or replaced.