Then something in your user account settings may be broken. Try to log in from a different user account to see, if Aperture is working, if you try to export using the Guest account.
- if it does not even work from the Guest account, reinstall Aperture.
- But if the Guest account works well, quit Aperture and open your user library. Remove the Aperture's user preferences and the Export Presets from the User Library as described here:
Aperture 3: Troubleshooting Basics http://support.apple.com/kb/HT3805
Note: Your User Library is hidden by default in 10.7.x or later - to open it in a Finder window use the "Go" menu from the Finder's main menu bar.
- Quit Aperture, if it is running. Log off and on again.
- Open the user library by using the Finder's "Go > Go to Folder" menu and hold down the options-key, until "Library" appears in the drop down menu. Select it.
- In the widow that will open, scroll down to "Preferences"
- From the "Preferences" folder remove "com.apple.Aperture.plist".
- From the "Application Support" folder remove
~/Library/Application Support/Aperture/Export Presets.plist
Then try to launch Aperture again.
But deleting the "Preferences" file will cause Aperture to forget the preferences settings. Be prepared to have to reset all options you set using the Aperture Preferences panel.