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.
Well, I tried repairing the permissions and then the database. I can't say yet whether or not it did anything, because I didn't always experience the problem that I was describing.
That said, when I repaired the permissions, it went through its routine and nothing. Upon repairing the database, it did recover one file. Oddly enough it was a video file which had been deleted weeks, if not months earlier. I still had that video file stored outside of Aperture, so I went ahead and deleted it again from Aperture. So far everything seems to be running fine, but only time will tell. I'll post back if I start to experience problems again.
Speaking of a working backup, I do have all of my images stored and kept as referenced images. I also have the masters backed up in multiple locations. I also have a vault of my library stored on an external disk and I have that vault backed up in multiple locations. I am assuming that should an issue occur, I could just use the vault to get my library back? I've always been a little confused as to how the vault works. Let's say I completely lost my Aperture Library. Could I just open the vault to restore the library to full working condition? My confusion lies in the fact that I have a referenced library, so I am assuming that the vault only stores information about the masters, and not the masters themselves. If I were to lose my library or something were to happen, would I have to first load all of my masters again and then open the vault? Would I load the masters, reference them and then open the vault, or would I just open the vault and it would take care of everything?
I also have the masters backed up in multiple locations. I also have a vault of my library stored on an external disk and I have that vault backed up in multiple locations.
That should suffice, if you keep your vault up to date.
From the Vault you can restore the Aperture Library - everything cointained in the .aplibrary package. The referenced masters outside the package you will have to restore yourself, if they need restoring too. If you have to restore them to a different location - maybe because a drive failed - you will have to reconnect the Aperture library with the referenced masters using the "File -> Locate Referenced Files" command.