That is a general error message, when Aperture cannot open an aperture library and either crashes or encounteres an error.
What is your Aperture 3 version? And what happenend, before you could no longer open your Aperture library? Did you import new images? Upgrade or install software? Move your Aperture library to a different location? Did Aperture crash? Where is your Aperture library stored?
Try the Aperture library First Aid Tools: