Let’s say the end of our civilization and going back to middle age or something like that.

The middle ages were in the middle of something called "the little ice age" which was entirely different and, obviously, mostly had the opposite effect.

The most dramatic story about that is what happened to the settling of Greenland (more than once). Basically during a cold spell the city was gradually abandoned, with sometimes the last few people literally freezing to death when their fuel ran out.