There are a couple of reasons why your app may not be appearing on the Store.
- Your Apple subscription has expired. This is a yearly subscription which needs to be updated - please log on to your account here
If your account subscription is up to date then you many need to improve your App Store Optimisation.
App Store Optimisation is a process of optimising mobile applications to rank higher in your apps stores. It is important to ensure that your app is visible so that your customers can easily find your app to download. As more people download your app it will 'bump' up the App Store and it will be easier to find. There are quite a few articles on the web. Here are a few choice articles
Here are some simple few options for you to try