Recover an app is the same as you install a new app, let's take `Smart Content Protector` as an example.
1. Go to the app Page, here is https://apps.shopify.com/smart-content-protector
2. You can see the blue button 'Add app', Click it.
3. A modal with a text input show and you put your shop domain there, for example, `content-protector.myshopify.com`
4. Click the `Log in` button, then you can re-install the app.
Here are some images for it. Maybe this description can help you.