The specifics reported in an Apple Support Communities thread, tweets to @applesupport and via email suggests that it may be nothing more than Apple’s activation servers struggling under the load, but some are reporting that their devices have still failed to activate 24+ hours later.
A couple of solutions have worked for some …
Some owners have found that simply connecting the iPad to iTunes has resolved the problem, while others say that this failed but a full restore via iTunes has done the trick. This will, of course, lose any changes to data since the last backup.
We’ve reached out to Apple and will update with any response.
One other reported iOS 9.3 glitch turned out to be a settings issue, with a simple fix for those find that Night Shift schedules were not available.