Here's the announcement from the website, which is dated over 2 weeks ago:
Ibis Reader will be shutting down soon and is no longer accepting new users. Existing users should download their books by selecting the “Download this book” link on the left side of any reading page under the table of contents.
When Ibis Reader was launched in early 2010, it was one of the first reading apps to be built in HTML5 and exist entirely in the web browser. This app predated Kindle, Kobo, and Nook efforts to develop a platform independent reading app, though it was later eclipsed by those commercial platforms. Ibis Reader was also the first reading app which could be installed on the iPad and iPhone that bypassed iTunes and Apple's strict control of the apps allowed on iOS.
It was a great idea at the time, but TBH I stopped using Ibis Reader long ago. There are now so many different ways to get a reading app on to your preferred device that there was little point in continuing to use a browser-based reading app. I suspect that is true for many, and that could be why the shut down happened so quietly.
Ibis Reader was developed by Threepress Consulting, a small tech company that was acquired in January 2012 by Safari Books Online. When that deal was announced Liza Daly, the principle at Threepress, hinted that Ibis Reader had an uncertain future and might not survive the transition to Safari as the Threepress team was integrated into that distributor's workforce.