What do you mean that boards run up and down but not across the middle? Surely there are ceiling joists every 24 (or 16)” in the attic. That is what the ceiling drywall should be nailed to for support. Is that not the way your house is constructed? Please post back with more info. If it was not nailed in the field, then just prop it up and screw it into the joists to pull it back in proper position.