How do I stop a product from being hidden?

Stop hiding products that are out-of-stock
You can tell Zero Øut to not hide an out-of-stock product by tagging it with 'zero_out_no_hide' from the product's configuration page in your Shopify admin panel. 

To stop hiding all the out-of-stock products without having to tag them one by one with "zero_out_no_hide", you can just simply hit the "Hide out-of-stock products" toggle off using the ZeroOut app. With this, all the products hidden by ZeroOut will again be visible in the store, if not configured in the Shopify admin page.