Let's game some options, remembering that these options are general. These actions aren't specific to the type of breakdown of society (external by an attack from a foreign nation, or internal from economic collapse, for examples).

So, we have our collapse. Let us "X" out a nuclear war/nuclear terrorist attack, as we can deal with all the other scenarios in variables without radiation to contend with. Let's identify the largest challenges faced for that high-rise apartment resident in Manhattan, or the family in the brownstone on the South side of Chicago. First, let's game the scenario:

After "The Day," the city was almost completely without power. You and your wife and two children were not able to leave town. All mass transit was halted or discontinued. It has been three days, and your family has been listening to static on the radio for the most part, with "campy" pre-recorded disaster broadcasts that have not been helpful or informative. One of your neighbors left this morning after saying goodbye: he and his family had a boat, and they were heading out of the harbor, hoping to use one of the major rivers to make an escape.