Or are we supposed to believe they ran out of bullets and gasoline?
They'll have bullets and gasoline, but if you put them in guns and cars respectively, they won't work. But throw gasoline on a fire, and I bet it works just fine.
A certain segment of the audience will say, "hey waitaminute
..." But most won't even think about it, and the show won't care. Much angst will be expended online while we wait for a good explanation, which may or may not be forthcoming.
I'm on record rooting for aliens because a) we can "explain" it as some sort of advanced alien ray-beam that zapped only the technology and b) it won't be very long till we're all totally bored by the family drama and need an alien armada just to keep our eyelids open.