Odd.
Try restarting the site. If you cannot restart the site, try opening the web.config file, make and undo a change and resave it (a changed web.config will cause it to restart the site. If you have ftp access, you can copy the web.config to your comp, add some small change (a space at the end of a line), undo, and resave. Replacing the old web.config on the server with the new with a new datestamp should force the server to reset.
Look at this thread: http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/118/threadid/148374/scope/posts/Default.aspx
If nothing there works, you could try.....
Can you navigate to the pages directly bypassing the menu?
My display may be different than yours (I may be using something to change urls IIRC).
As you can see the tabs tend to be lower numbers since host and admin are usually early added tabs. And they increment.
My default.
http://localhost/Admin/SiteSettings/tabid/39/Default.aspx
http://localhost/Host/Host%20Settings/tabid/16/portalid/0/Default.aspx
Portals.
http://localhost/Host/Portals/tabid/17/portalid/0/Default.aspx
Module defs
http://localhost/Host/Module%20Definitions/tabid/18/portalid/0/Default.aspx
FileManager
http://localhost/Host/File%20Manager/tabid/19/portalid/0/Default.aspx
Then
http://localhost/Host/Vendors/tabid/20/portalid/0/Default.aspx
http://localhost/Host/SQL/tabid/21/portalid/0/Default.aspx
http://localhost/Host/Schedule/tabid/25/portalid/0/Default.aspx
http://localhost/Host/Languages/tabid/27/portalid/0/Default.aspx
http://localhost/Host/Search%20Admin/tabid/29/portalid/0/Default.aspx
http://localhost/Host/Lists/tabid/33/portalid/0/Default.aspx
http://localhost/Host/Superuser%20Accounts/tabid/34/portalid/0/Default.aspx
http://localhost/Host/Skins/tabid/35/portalid/0/Default.aspx