If there is a Windows Performance Counter that reports the status of the server, you could capture it as a metric and use that metric in a health rule. For example, there is a Windows Performance Counter that reports the current status of an IIS application pool:
Performance Category Name: APP_POOL_WAS
Performance Counter Name: Current Application Pool State
Instance Name: The IIS Application Pool you want to monitor
Values: The possible values are 1 through 7. 3 means the app pool is up, 5 means it's down.
Someone wanting to know when an IIS app pool is down could monitor this performance counter as a metric and create a health rule that violates if the value is >4 and <6 (aka =5 - there is no equal comparison operator in health rules).
An easy way to add the performance counter is to download the Windows Performance Counter - Configuration Extension, run it, find and add the counter, then restart the AppDynamics.Agent.Coordinator service. This configuration extension can be found here:
You could then create a policy when the rule violates and another when it stops violating.