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) that messed with Earth's magnetic field. (Hard to work that one without killing off all the people.)
Or, the growth of a religious cult which declares all technology to be the Devil's work and have the ability to destroy power plants and to brainwash people into willingly accepting a more elementary life style.
That's not too far fetched when you consider the way ISIS have destroyed many ancient historic sites because they consider them not worth of Allah, and when you look at old movements like the Luddites (although their motivation was the fear of unemployment rather than religious).
