That may have many causes, so tell us more about your system.
What is your Aperture version?
Is your Aperture library managed or referenced? On an external drive or your system drive?
What did you do, before this freezing first happened? Did you import new images? Upgrade software or install someting new? Plug-ins, programs? Move your Aperture library?
Anything what changed at that time? Are any other programs having issues?