This is quite strange. I think a couple of things might be happening:
Possibility 1: You are in Side Note mode. In this mode you should be seeing
only the page surface and toolbars, but no page, section, or notebook tabs
around your page surface. Side Note come up when you click the OneNote icon
in the windows taskbar (near the clock). If you want to start OneNote in
normal mode, right-click on this icon instead, and click Open OneNote, or
just open OneNote from the Start menu.
Possibility 2: Your toolbars and page, notebook, and section tabs are still
visible, but the menus disappeared for some mysterious reason. You could try
restoring the normal conditions by deleting your Toolbars and Preferences
files. Here is how to do that. Close OneNote. Go to C:\Documents and
Settings\%username%\Application Data\Microsoft\OneNote\12.0 and dete the
Toolbars.dat and the Preferences.dat file. Note that this means that you may
have to loose your note flag customizations and recustomize them again.
Hope this helps,