Have you checked, if the missing masters are inside the Aperture Library package?
Sometimes Aperture looses the connection to managed masters.
You need to find out the exact date that you imported your images - you can find that information in the "Import Session" metadata tag - ad this tag to one of the metadata preset that you use to display the metadata in the info panel - set themetadata preset in the info panel to "edit".
Then open the Aperture Library package (ctrl-click the Aperture Library in the Finder window) and select "Show package Contents". Inside the window that opens go to the "Masters" folder, then inside to the year , month, day you imported the images. Are the original master image files still there?
If yes, then make a backup copy of these images, to have a copy just in case, and try to repair the permissions in your library with the library first aid tools:
Aperture 3 User Manual: Repairing and Rebuilding Your Aperture Library
If you cannot reconnect the missing originals/masters this way, reimport the masters you found inside the library.
Please post back, once you looked inside the library, if the masters/originals are still there.
Regards
Léonie