End of the world ideas

As did Alistair Reynolds in Century Rain. Humanity was forced to live offworld after a "grey goo" type nanobot disaster.

 

How about some kind of super powerful Tesla coil? Fun fact: Nikola Tesla wanted to build one so powerful that it would never have to be used, aka total destruction. Maybe someone built that weapon, and then it was accidentally fired.

 

The way you said it doesn't sound scary, but if you imagine a picture of it, that's scary.

 

In addition to what @soupcannon said, you could have a solar flare causing an mass terrestrial EMP spike that permanently knocks out all electricity, power stations, cables, television, telephone, planes, automobiles, computers everything that relies on electricity hey presto back in the dark ages!