Have you done what the dialogue box says? Go into aperture and enable sharing with other applications, aperture->preferences->previews-> share previews with other applications ->always.
How can you google this and not find it? Well, you haven't really explained your issue fully. Did you actually put your aperture library back after the reinstall? Basically, your iPad is looking for your aperture library, and it can't find the previews that it uses to generate the images it syncs to the iPad.
How did you reinstall? Via time machine? Time machine does not back up the thumbnails files within an aperture library, so it's possible the thumbnails are missing and need to be regenerated.
There could be lots of reasons, but you have to explain first what it is you're doing.