Good commentary and analysis of Everyblock's bizarre instant-shutdown.
It appears that NBC has made the decision to shut down EveryBlock. EveryBlock was originally funded by a grant from the Knight Foundation, and when that grant ended it was acquired by NBC. That, somewhat controversial, move may have pushed the Knight Foundation to rethink some of its grant terms.
Given that history, it seems especially unfortunate that this community resource is now shut down with no warning, and the data that the community worked hard to create is now (apparently) gone for good. I hope NBC at least releases the community-created data for others to use in the future, but I doubt this will happen.