Some apps on Heroku may be automatically archived.
When archived, an app won’t be accessible via any web domain. In particular, accessing the app on its .herokuapp.com domain will yield a “No such app” response.
Currently apps are archived only if they have never had a user-initiated deploy, i.e. they are empty apps or apps created from templates and never changed.
Use the Heroku CLI to determine whether an app has been archived:
$ heroku apps:info -a example-app
=== example-app
Archived At: 2013-07-24 01:10 UTC
...
Apps can be un-archived by deploying any change to the application. See the getting started guides for details.