The Wealth of Nations

What happens if nations stop trading with each other? It depends on the knowledge, resources and people of any particular nation. A nation can maintain as is, move forward or 'forward to the past':

Most nations can revert to this. Back to farming (but probably not hunting coz not many furry friends left). It will require seeds, tilling etc though.

Next up is the mining of ores and smelting of the same to metals to make tools. Many nations can probably do this rudimentarily if they have the ores but it won't be straightforward because working with iron requires very high temperatures.

It is possible to bypass or minimise metal usage. Some nations have or used to have sophisticated weaving, carpentry, woodworking, and timber-based construction skills. Other nations used to have advanced stone working skills too but these are mainly lost.

This requires the harnessing of metals, fuel and fire to make engines. And to use these engines for mass production. The car is a prime example of a product of the industrial age. Making a car is not as easy as people generally perceive (particularly the engine and transmission), it requires lots of maths, metallurgy, mechanical engineering, manufacturing and logistics skills.

Initially transistors, silicon chips are the basis of the gadgetry and wizardry of the modern world: phones, computers, routers, internet etc. Transistors are probably still doable by some nations but modern silicon chips can be made by only a few advanced nations.

Wealth of Nations
Where nations will end up in if they don't trade with each other depend on the depth and breath of knowledge kept by their universities, institutes, businesses, guilds, individual experts, hobbyists and traditions. These will determine if a nation can 'rebuild from scratch' or forge an independent path.