If a product you previously had live on Amazon is now showing as closed, it's because of one of the reasons below.
You applied a Sale End Date or Offering Release Date on the product. Once that date passes, Amazon automatically marks the listing as closed.
From the Product Linking page, on either the Linked or Unlinked tab, the bulk action Close Listing sends the selected SKUs to Amazon to be closed. If you (or someone else on your team) ran that action, the listing was closed on your request.
The app closes a listing automatically when you have Deactivate product on Amazon enabled. This setting is configurable in two places:
With that setting on, the listing is marked Closed on Amazon whenever you, in Shopify: