It is unusual (in my experience) that restarting didn't clear the problem. Here are the next two attempts:
Close Aperture. Go to "System Preferences→Displays" and change the display size. After it changes, change it back. Restart Aperture.
If that doesn't work, start your computer in Safe Mode. Then shut it down and restart regularly.
The suggestion by Kirby didn't work: quitting Aperture, changing and rechanging display size, and restarting Aperture.
What did work: change display size while Aperture is running, quit Aperture in that setting, reset the display size to the default, restart Aperture. Everything is back to normal now. No clue, however, how on earth my daughter got me in this situation...
Removed com.apple.Aperture.plist and rebooted Aperture. You'll need to relink your library after this, but otherwise it's perfect!
If you have this problem, see Kbase article HT3805.
You can resolve many issues by restoring Aperture back to its original settings. This will not impact your image files or your library. To reset your Aperture user preference settings to their original state, do the following:
- Quit Aperture.
- In the Finder choose Go to folder from the Go menu.
- Type ~/Library/Preferences in the "Go to the folder" field. Press the Go button.
- Remove the "com.apple.Aperture.plist" file from the Preferences folder.