Google Wave has been dead for quite a while. But not as dead as now. It was only a few weeks after the service was made available to everyone that Google announced it would kill it. Euthanasia though takes a long time for something like Wave.
The technology behind the project was open sourced and became part of the Apache Software Foundation, dubbed Apache Wave. But the Google Wave servers remained live allowing existing users to continue to edit waves or create new ones.
It would not last though, Wave has been in read-only mode since January. At the moment, the site is still accessible, though read-only, but Google had an April 30 deadline planned.
Any moment now the site will be shut down for good. If you haven't exported your data, there's still some time left. Of course, anyone truly interested in the project has moved on to the open-source alternative by now.