Question
How can I use AppD4DB to alert if a database instance goes down? This is not for when an OS goes down. We want to know if SQL Server crashes, for example.
Answer
Set up a Custom SQL Alert like this:

If the SQL Connection is successful and it can run a query then it will return a 1. If it fails then you will get an email containing the exception.
For example, here is the alert history:

This is the contents of the default alert:
