How to Isolate a Modern City

My setting idea begins with modern day Earth. At some point in the not very distant future, someone begins messing around with magic--perhaps in the form of superscience, perhaps as Ancient ritual, who knows. The point is, magic is awoken--but magic here is fed by the Void. It starts eating away at the world the more it is used.

The setting in which I want to tell my story is a large city--perhaps New York--which has been cut off from the rest of the world by the Void. They have been on their own for at least a few decades, probably longer--I don't know yet if I want there to be people who can remember a time before the city was cut off. If so, it should be a distance memory. Magic use is forbidden outside of very special circumstances, mostly powering devices the city needs to stay alive and catching other magic users.

My issue is that I need to figure out what this city would realistically end up as if it were cut off from the rest of the world. A few things I can already tell--there would be no cell phones, for example, but people might be able to hook up landlines. It is unlikely that their reserves of fossil fuels would last for long, so there probably aren't cars still in use. The power grid would still be in place, and they could probably provide electricity either via solar power or limited magic usage.

But, what about food? How much land space would a city the size of New York need to provide food for itself? How many farmers? How many bullets and guns would be inside the city, and how capable would people be of making more? What things do we take for granted that would disappear without the outside world? There's a lot of logistical questions to answer, and I need advice.

What do you think this city would be like?