You may have an invisible filter set. This can be cleared by the method described in Frank Caggiano's User Tip:
Images not appearing in browser, search filter is cleared
If that does not work, I'd try to repair or rebuild the Aperture Library using the First Aid Tools:
Launch Aperture with the cmd-option key combination (⌘⌥) firmly held down. From the "First Aid" panel that appears try the three options: Repair Permissions, Repair Library, and Rebuild Database in turn. Hopefully one of them will bring your images back.
Yeah, it looks like if the images are hidden in iPhoto you will need to unhide them if you want to see them in Aperture.
They've described the workaround in a kb article.
From the kb article:
Photos hidden using iPhoto's Hide command cannot be accessed in Aperture. If you open the library in iPhoto, unhide the photos and then open the library in Aperture, the previously hidden photos are accessible.
I recently had the same issue and did the following to correct it.
1. Open iPhoto and create a new Smart Album.
2. Set the criteria to "Photo" "is" "hidden."
3. In order to export these photos, they must be unhidden, so set some other attribute that will keep them unique from the rest of your library, such as flagged or 1-star (you will have to un-flagged all other items in your library to keep these hidden photos sequestered).
4. Now that they are all flagged or have 1-star, highlight them all and unhide them.
5. Change the criteria on the Smart Album to flagged or 1-star.
6. Highlight all the images, click export. You can export the originals to your desktop with their event names as folder names, and simply drag and drop them into the placeholders in Aperture.
It was a pain but it did the trick.