Super + W used to zoom out and show all open windows and workspaces, but it has stopped working.
How can I re-activate that?
You need Compiz Config Settings Manager, you'll find it under Window Manager > Scale > Bindings
What changed between the time it worked and now, that it is not working?
This is important information to have.
Super + W doesn't work under Unity-2d, so maybe you're running Unity-2d. You could try logging off and selecting the "Unity" session prior to re-logging in. I can't give you more specifics without knowing which version of Ubuntu you're running (I'm guessing 11.10, if so, click on the little cog that's just above and to the right of the username in the login screen).