Here's the (hypothetical) scenario: I've updated my TeX distribution in some fashion. Now all of a sudden, a package that used to work just fine is no longer working. Of course, the list of what could have gone wrong is endless, but I would expect there to be a few common things that one should check first before trying to dig too deeply. So that's the question:
What are the common causes of a package failing to work after an upgrade, and how does one diagnose and cure them?