That is typically the case, when one image in the Library is corrupted and Aperture cannot render the version from the master image file.
If a single image is the culprit, you should be able to export a test image. Import an image for testing, that you have checked in Preview. Try to export that singel image. Does it work?
If you can export new images, try to find the corrupted image or video.
Launch Aperture with the Shift-key held down to prevent it from trying to render previews and stalling. Then try to manually render previews for small groups of images from your most recent import.
To manually render previews select the images in the browser and use the command "Photos > Update Preview/Generate Preview" from the main menu bar.
Aperture will stall again, when your current selection contains a currupted image original or version.