Now, that would be rock bottom wouldn't it?
If by "rock bottom" you mean "a thousand years of darkness", then yes.
I don't know if it would take us a thousand. We'd have the rest of the world to keep technological knowledge, etc., alive.
Assuming the resulting economic devastation doesn't drag everyone else down with us.
Oh, everyone else would face economic disaster as well. Simply not likely civilization-ending as what would be occurring here.
Very plausibly we'd lose about 2/3rds to 3/4ths of our population, though. We'd cease being "the United States of America" for some time.
Best case scenario.
Depends. We grow lots
of food in this country, and many people would likely be able to flee to neighboring nations.