What happens if my Apple certificate expires or has been revoked?

Apple Push Notification Service Certificate
You can no longer send push notifications to your app.

Pass Type ID Certificate (Passbook)
If your certificate expires, passes that are already installed on users’ devices will continue to function normally. However, you will no longer be able to sign new passes or send updates to existing passes. If your certificate has been revoked, your passes will no longer function properly.

iOS Distribution Certificate (App Store)
If your iOS Developer Program membership is valid, your existing apps on the App Store will not be affected. However, you will no longer be able to submit new apps or updates to the App Store.

iOS Distribution Certificate (in-house, internal use apps)
Users will no longer be able to run apps that have been signed with this certificate. You must distribute a new version of your app that is signed with a new certificate.

Reference: Apple