Is there a way to get some notification when a Cloud Foundry application fails or is unreachable? I mean to register to some deployed app and if the status of the application is changed to failed or something, I want to receive a notification.
On Pivotal Cloud Foundry, when a app crashes, an event is emitted thru the firehose.
PCF Metrics tile, available from Pivotal, can be deployed to your PCF foudnation. PCF Metrics will track all events for apps running on the foundation and are accessible to developers (thru Apps Manager). I believe Metrics tile tracks history for up to two weeks. I am not aware of any alerting capabilities in the PCF Metrics tile (I could be wrong, in which case, please correct me), that will prompt you when an app crashes.
Other approaches are to implement event logging tools like Splunk, New Relic etc. They support alerts. You will have to build those.
API monitoring tools like AppD, Apigee, and New Relic provide alerting and can notify you went the response time to an app has degraded (as in your app has crashed). This approach is a little more involved. You may require to add an agent to your buildpack, depending on the tool you choose.