If all of sudden your app no longer shows in the App Store, chances are that your Apple Developer Subscription has expired and needs to be renewed.
Apple charges a $99 annual fee for the Apple Developer Program, which is required in order for the apps you have on your account to remain accessible through the App Store.
In order to have your app reinstated all you need to do is renew your Apple Developer Membership.
To confirm that this is really the problem you can log in to your Apple Developer Account here: https://developer.apple.com/account/
If your membership is expired you will see a red alert displayed at the top:
To resolve the problem make sure you are logged in as the account owner, you will then see a button that says "Renew Membership".
If you are not seeing the "Renew Membership" button it is likely that you are not logged in as the owner of the account, in that case, you will need to contact the owner and ask him to renew the membership for you.
If you are still having problems contact the Apple Developer support team through this link: https://developer.apple.com/contact/topic/select