Since your server is up I would in general consider two possible points of failure. First I would check that the collector this agent is connected to is up and running with no issues before looking at the agent/application. A quick check of the logs to make sure no connections are being refused or anything like that would also be good.
If you don't see anything on any AppMon components then I would have the application owners verify that the agent is still installed and configured properly (I'm aware of cases where it has been removed without notice) and then check those agent side logs to see if there are any issues during startup. Of course the agent will only be connected when the application is running so make sure this is the case as well.