you do not mention checking the filters, so just to be sure - have you checked the filters in the Browser and the Library Inspector, if all filters are cleared and no items excluded? and also checked for Invisible filters - see Frank Caggiano's user tip:
Images not appearing in browser, search filter is cleared
And have you tried the other Trouble Shooting Basics http://support.apple.com/kb/HT3805?viewlocale=en_US&locale=en_US, removing Preferences?
If none of this helps, make sure you have a backup of your Aperture library and try, if you can rebuild the library by creating a new, empty library and importing your current library into it.