The most common reasons for this are either a corrupted library or a corrupted image that you recently imported, and Aperture is not able to build a preview.
Thy the Aperture Library First Aid as described here:
Aperture 3: Troubleshooting Basics: http://support.apple.com/kb/HT3805
Start with repairing permissions, then repairing the library. If that does not help, try to remember which images you imported last and inspect the previews - see if one of thoses looks corrupted,
Post back, if repairing the library does not help.