If you run a trace to capture failed logins, you'll get all sorts of extra information, such as the name of the application that's trying to connect (if the application specifies its name in the connection string). But it must be something that was connecting all along, and is only failing now because the database has been pulled away from under it. You'll need to consult the documentation for the application that uses the database, or speak to the people that support it.

John