How much is "downloaded some pics from". Can it be that the colored beachball has been spinning because Aperture was busy with raw processing your recent import? Ot you may have imported a corrupted image, that now prevents Aperture to launch, because Aperture cannot process it.
The link I posted explains nicely the possible trouble shooting measures.
Aperture 2: Troubleshooting Basics: http://support.apple.com/kb/HT2945
I'd start with the paragraph:
If Aperture does not open, it may help to defer creation of previews. In Aperture 1.5.2 or later, press the Shift key immediately after you start Aperture to prevent preview generation for that session. If a damaged image in your library is preventing normal opening, this may allow you to start Aperture.
If that does not help, try:
Press and hold the Command and Option keys while you open Aperture. The following dialog should appear:
If necessary, do a rebuild.
Report back, if this does not solve the problem or if you have more questions.
Good Luck
Léonie