I don't know what causes this, but it seems to happen quite frequently. When it happened to me, reinstalling fixed it, but this is how I do a reinstall:
Move my Documents/Caw/User Tool Data Folder to my desktop
Move my Programx86/EA/Caw/Packages to my desktop, if you have one.
Clear out my virtual files, instructions HERE
Clear my game caches, instructions HERE
Run a registry cleaner, I use CCleaner, HERE
Reinstall a freshly downloaded caw, just in case the previous one got borked in the download.
I just spent the evening on skype with a friend who had the same problem. We finally got her caw working by removing the sims 3 folder from Documents/Electronic arts, and restarting the game to generate a new folder. Caw opened and worked properly. At this point we started replacing the items in the newly generated Sims 3 folder, with the items from the old Sims 3 folder we took out. First we put back all the single items under the last folder. Opened Caw, and it still worked. Closed caw replaced each folder, restarting caw each time to verify things worked. We saved the DCCache for last, because that was the most likely suspect since she didn't have a Mods folder, and that was the problem. She reinstalled her store content, and all was fine.