Nellisir
Hero
Comparing this to our real, modern, world, it would be like if you have any kind of commute outside your immediate town or city, you are suddenly completely disconnected from home and family. Forever.
The closest quasi-realistic scenario I can think of would be a complete failure of the internal combustion engine. On one hand, it would be catastrophic for modern civilization. On the other, we're only rolling the clock back a hundred years or so. 1900 wasn't exactly the Dark Ages.
Past that, you're looking at force fields or space/time dislocation or something.
transportation: gasoline/oil runs out so the bulk of all transportation is shut down. No cars, trucks, trains, airplanes are running because everybody lacks the fuel. Riots over fuel prices cause social disorder initially. Folks with horses/oxen start hauling supplies, slowly. Alternative fuel sources are invested in, but adoption is slow because the materials and end products can't quickly be delivered.
People power will be the big thing for some time. We just don't have the animal/human ratio we had a hundred years ago. Manufacturing will shift to be more localized, which is great for the Northeast (relatively mild climate; high population; extensive manufacturing history to learn from/exploit/revive). Rivers and railroads become primary means of bulk transportation once again, though the interstate network & paved roads make bicycles & steam vehicles faster and smoother than they were in the beginning. Ocean transportation takes the biggest hit; very, very, very few people can build a ocean-going sailing ship nowadays, and steam vessels are a) not common either, and b) reliant on coal for fuel, which will have mining and transportation issues.