Diddly Doo
Diddly Doo thanked Didge Doo's answer

Hello, Diddly Doo. Are you and I related? Pleased to meetcher. :D

I guess you're writing a story and want a scenario in which we've been thrown back to the dark ages.

Two possibilities come to mind: A space event, such as a near miss with a comet (which itself would need to have some special qualities) … Read more

Diddly Doo
Diddly Doo voted up Walt O'Reagun's answer

Soul Fly gives a good example of a natural disaster.

A "man made" cause would be widespread use of EMP.  A few devices set off in orbit would have the same effect.

And without electricity, I don't doubt there would be widespread death in developed/industrialized nations. 

Just the lack of air conditioning in the summer, or … Read more