What does "Due to inactivity for 15 days, this app has been deactivated." mean? How do I reactivate my app?

If there are no messaging events(incoming or outgoing) activities for 15 days, the app is considered inactive and is deactivated. Once deactivated, you will not receive any incoming message events for the app.

However, you can reactivate your app instantly. All you need to do is click the reactivate button from the settings page of the app dashboard, as shown below.