The just-in-time supply chains that keep the lights on and food and other goods in the stores would fall apart within a couple of weeks.
Medieval starvation was local. Areas were largely self-sufficient in the bare essentials - except in bad years, which happened regularly enough to keep a lid on population growth, but not so regularly they caused widespread depopulation.
Trade was mostly for useful extras.
That's very much not true today. Everything is specialised and interdependent, and not even farming areas are truly self-sufficient.
This was - and possibly still is - the biggest risk from a pandemic. It's hard to calculate the exact tipping point, but when you have a certain percentage of the population unable to work for a couple of weeks or so, essential systems start breaking down.