We had a custom status named "Blocked" which was a custom open status for Maniphest. I personally disagreed with the existence of this status and we only had a relatively small of tasks using it. As such, I decided the remove the custom status, restoring maniphest.statuses to the default value. One unintended side effect, however, was that all tasks which were previously in the "blocked" status became closed.
As a quick fix I just ran UPDATE phabricator_maniphest.maniphest_task SET status = 'open' WHERE status = 'blocked', but maybe there should be a cleaner way?