You have taken your primary metadata controller down for maintenance, and your backup controller has...

Just turn it on. It will automatically retake its position as primary controller.

Turn it on, and then use cvadmin to manually fail the volumes on the backup controller.

Turn it on, and then use Xsan Admin to manually reassign it as primary controller.

Turn it on, use Xsan Admin to reassign its failover priority to "high," and then restart it.