The world has suffered a magic apocalypse. We pushed technological progress too far, and now magic has returned with a vengeance. It comes in waves, without warning, and vanishes as suddenly as it appears. When magic is up, planes drop out of the sky, cars stall, and electricity dies. When magic is down, guns work and spells fail.
My favorite part of this premise is that it implies that tech is just another type of magic, basically.
