Hello Long,
Thank you for your prompt reply! I appreciate your confirmation that the problem is not with MetaBox. I have checked the plugin and can confirm that the problem was created by "Custom Post Type Changer" here: https://wordpress.org/support/plugin/custom-post-type-changer/reviews/.
My posts came back when I re-created the same custom post type slug, so that's a relief.
Cordially,
Nadine
