Google announced a while ago that they were going to shut down their Google Wave project.
The company has now turned the service in read-only mode. This means you can no longer update your waves or start new ones.
Google Wave still offers you to export your existing waves for backing up.
Google is suggesting that you backup your important waves before the service is shut down permanently.
The company said:
More than a year ago we announced that Google Wave would no longer be developed as a separate product. Back in November 2011, we shared the specific dates for ending this maintenance period and shutting down Wave. Google Wave is now in read-only mode. This is a reminder that the Wave service will be turned off on April 30, 2012. You will be able to continue exporting individual waves using the existing PDF export feature until the Google Wave service is turned off. We encourage you to export any important data before April 30, 2012.
Folks who still want a similar app can check out Apache Wave. Google had contributed this project to the Apache Foundation.