I don't really know exactly why it happens, but reinstalling does help most of the time.
Follow the steps here on how to manually uninstall Ambitions: http://help.ea.com/za/article/manually-uninstalling-the-sims-3-ambitions/
Then reinstall the game like you did before.
EDIT: I merged all your posts together. I understand you're frustrated and desperate for help, but please don't double post. We're just users like you here and do not work for EA, which means we help out in our free time.
