Application Agent Is Not Running -yet another case!

I have not yet read a thread in which the nature and causes of this problem have ever become clear to me.

As well as the application agent not running, CFW is behaving as though it were newly installed again. I’m having to teach it all the same stuff. Does this mean there’s a profile in Application Data that is broken?

How should I go about diagnosing the problem?

What usually causes it, and what is it that the non-running of the application agent cause to be wrong? Is there a definitive answer to this?