I've had to design stuff for post apocalyptical worlds and medieval worlds so I imagine this stuff all the time. I imagine EMP would be the scariest apocalypse if you destroyed all the machines that make the machines then we'd be thrown back around 2 - 300 years in terms of development with very few people capable of making things with pre-electric tools.

Scorched earth is a whole different kind of scary though the worst has to be the medieval turn back type apocalypse such as depicted in the books by Mark Chadbourne. The idea of being thrown in to a realm 500 years out of date with no idea how to survive it is pretty bad.