Then they start making stuff themselves, which is what the protectionists wanted.
Beyond that, I don't know. A lag in growth from not being able to buy new machinery from Europe could be offset by importing fewer finished goods.
Loss of export wealth would have cut into the luxury market, but Northerners did have the resources to produce what they needed at home and they weren't dependent on something like foreign oil to survive.
They were self sufficient as far as that went, but cutting 200 million out of your economy in 1861 would have caused a financial panic. It would have slowed down growth and probably caused a big upspike in unemployment.
My point is, the financially powerful people of New England did not want the South to secede because it would crush much of their finances, and their former customers would become competitors.
No, Southern secession was a dire financial threat to the New England economy.
Actually we do know what happens - happened. There was a dip in production, in trade, in every way measurable. But most of the losses were regained over time. This crap about the north couldn’t survive without the south was just that - crap.