Even with the energy question laid to rest, you would still need raw material such as lumber, steel, uranium, albumin (used in the proccess that makes aluminium, I don't even think you have any of that naturally), the NA fisheries are in terrible shape, and all those types of things.
Let's make the question harder though: Assume that the US has all these things (by invading the North, perhaps?). What would happen if your economy closed, but was self-sustainable.
First, your products would become more expensive. The people in NY that buy cars made in Oshawa Ont. do so because it is chaper to have them made up there than in GA or WA. So, even keeping thier incomes constant, they would not be able to afford a new car. They'd have to choose between a replacement toaster or a new winter coat, instead of just buying both. Chicken, not steak. Car pooling, to save on gas. New, cheap techniques to overclock Vid cards would come about. An overall drop in standards of living. And let's not forget what business would have to put up with. They can't upgrade thier networks, revamp thier production lines or buy that shiny new Deere tractor as easily. That means that their workers would not become more productive. Which means that the businesses couldn't afford to pay them more money. This would slow the rate of growth of productivity in the long term.
Now, look on the supply side. Why does that seattle company those widjets to Vancouver, and not FLA? Because they can get paid more in Van. If they could only sell in Talahassee, they would get less money for it. That means that they would scale back production, because it wouldn't be worth it to run three lines at that price, you couldn't cover your overtime expenses for the graveyard shift. That means workers unemployed. That means (since it would happen everywhere) that there would be even less demand for the products. Note that this same thing happens to companies investing in capital. The extra money they might have put into that 3rd Transport Truck isn't worth the interest rate (even if they stayed reasonable) you'd have to pay on the loan, because you won't make enough money.
That's the long-term effects. In reality, all that coming together at once would likely be a "Perfect Storm" which would send the economy into a recession. Which could mean that it could be much worse than the scenario I painted above.
And then, what about all the assets owned by foriegners? Would you just take them? You couldn't pay fo them, your dollar would be worthless internationally. The foreigner's couldn't sell them, because you wouldln't let any productive result of those assets out of the country. What do you think would happen to American-owned assets when that happens? They would likely be expropriated by other gov'ts just in order to pay off what you (as a people, not just a gov't) owe them, for taking their land.
Hm. . . and how many people do you think would come up to Canada, or escape down through Mexico into Europe and Asia? Alot of people have Human capital that are worth more in other countries (all of a sudden) than your own. Doctors, engineers, journalists, top managers. All kicking back in London, or Rio. Making a decent lifestyle. While your country desparately needs it's best an brightest, some of them will by flying away.
Unless you stop them. Land of the free and all that.
If that scenario wouldn't screw my own country as much as yours, I'd want to see it happen, just to see the experiment.