First off, do be sure Aperture supports the file you’re looking at. You can find a list of all supported RAW formats here.

Assuming the image is supported, there are a few things you can try, in no particular order.

• Restart the app — surpisingly that often fixes it
• Repair/rebuild the database
• If your Originals and/or Library are on an external drive, try a different FW or USB cable