If you have not done so already, try the Library First Aid Tools.
Has the file size of your library changed, after the upgrade, or is it still big enough to hold all the images? Probably the images are still all there, but hidden or not linked to the projects.
Post back, if repairing and rebuilding does not help.
I had already tried to rebuild the library, but i could only do it through aperture 2, cause as soon as i opened Aperture 3, it started rebuilding the library, and deleted everything. The files might have been somewhere temporary, cause there wasn't space for copying a new version of the 260gb library back to the internal harddisk, even though the finder showed 340 gb free space. Tried grand perspective (app that shows you a graphic version of your harddisk) and it didn't show it either.
Third time i didn't rebuild every image, but only imported the files. Guess they have to be rebuild if you want to use every tool on top.
I had already tried to rebuild the library, but i could only do it through aperture 2
The Aperture 3 release notes strongly advise you to rebuild the Aperture library before upgrading to Aperture 3. See the link below. You have to scroll down a long way, before finding this piece of advice.
Unfortunately, most people discover that the Aperture Library is corrupted only after upgrading, and than it will be too late - tfor hen they have no longer the previous version of the program and a backup of the library, so that they can go back and rebuild the library before upgrading.