Quite often on Apple's Universal Access forums there are questions from users who have found that their computer's suddenly started talking to them, and putting a black box around whatever they're working on.
Here's what the black box looks like in the Finder when I have the "User Guides And Information" folder highlighted:
What's happened here is that the user has accidentally triggered VoiceOver by pressing command-F5.
You can turn it off again by pressing command-F5, which works as a toggle.
Note that if you have a MacBook you might have to also hold down the "Fn" key at the same time, depending on how you have your laptop set up. Try it both with and without the "Fn" key and one of them should work!
- Ricky Buchanan