Somebody above stated the main problem, large cities and their inhabitants. When el, water, heat, gas shuts down the city areas tend to turn uninhabitable fairly fast or faster depending on the time of year. I believe most people living in them are quite helpless when that happens.
I remember too well when Soviet Union was falling apart, just in case the social order would fall apart (in the St Petersburg area) and people would start to move somewhere close where there were things like food (Finland), every vehicle in the Army that could be equipped with a machine gun, preferably a heavy one, had one. Fortunately no one had to give the FIRE-order. We were thinking of millions of people changing places. OK, how's London?