Subtropical, like South Eastern US or Eastern Australia.

Current estimates imply the Apocalypse occurred 100+ years ago. This is long enough that 2 or more generations have been born since the event, and it is relegated to oral tales and half remembered facts (or otherwise). The most likely number is probably about 150 years but we've been reluctant to put a hard date on it yet.

The short answer is, a global collapse of infrastructure due to several factors that happened in a cascade fashion. If you think about it, if one major component of the world's infrastructure fails (say, the entire power grid or a major portion of it) then what remains is going to be hypertaxed and things are bound to fall down like dominos if the problem is so widespread that it causes such a failure.

The general idea is “something” or “somethings” happened, causing simultaneous global outages and different ripple/side effects. Some areas fared better than others, but nothing was left totally unscathed.