Like in the title said: What would happen if every country forgot about the existence of other countries and there is a huge force field of every border of country till space.
The force field is 92.96 million mi tall I guess. So, with this force field they are invisible to each other and all communication is lost; they will forget the existence of their relatives who are in other countries, too.
So: How will the countries survive and advance in technology and other things?
It would depend on the country.
A country like Singapore that is basically a city state would suffer an immediate and catastrophic famine. Likewise countries like Saudi Arabia that have large food deficits and no means to overcome them.
Large countries like the USA with a big agricultural base and broad economy would be fine - indeed, after adjusting for oil imports and consumer goods, the USA might cease to notice. Other industrialized countries would have immediate problems with energy (Japan especially) but generally - the scientific knowledge to overcome problems would be present in all advanced countries and it would just be a question of keeping basic services going long enough to apply it.
You have to wonder what would happen in the event of a successful Scottish independence vote.
I think that the amnesia aspect is more troubling. For a country like the UK, history and culture are intertwined with the neighbors; vanishing this out of existence would leave huge and strange holes in many places.
Generally...
If a country does not collapse entirely, then first you'd see food and/or petrol rationing in most countries. Factories would go up to provide substitutes for imports - for example, we'd know that smartphones existed in countries that didn't make them, but it would take a while to reverse-engineer them. Imported consumer goods would become precious as they wore out.
This question covers a lot of information - too much for one answer to fully go into - however a list of all import and export by country is available here. With a map you can get out of that data looking like this for imports and this for exports. This shows we're all pretty reliant on each other for goods, services and entertainment.
Some countries build their economy around exports (china, for example) and so there would be HUGE job losses if they lost contact with other countries - probably leading to revolts, homelessness and require a complete rethink of how the country works.
Large countries would have more room for farming - though some (like Russia) aren't all useful land. They could probably sustain a larger population than the smaller countries.
A lot of western countries have lost expertise in manufacturing - Britain once had the world's leading steel industry and now that has mostly moved abroad. These countries would struggle to maintain the high standard of living they currently enjoy and would have to act quick to train their (perhaps reluctant) population in those skills they lost.
Another aspect depends largely on where you draw your borders...the vatican city (technically its own country) would have no hope of surviving if suddenly isolated. With split borders (ie Alaska and the US) they would have to be joined by the ocean.
Also countries often band together to see "Them" and "us"...so when there is no more "them" on the outside I think we would start to see an increase in the number of civil wars - especially in large countries like China, Russia and the US (the US may well split up in to separate states).
All in all it is fairly unpredictable...but I don't think anyone would come off well, more of the population would have to go into maintaining the country and population and much less into furthering research. Given that I think all countries would, for a while at least, stagnate.
(I am ignoring the science-based tag, I don't think there is any science done on this subject)
I assume you would fix all maps, all encyclopedias, all newspapers, all holiday photos and everything else. There would be A LOT of references to fix.
Many companies would suddenly find themselves without suppliers, customers or both. A multinational company has its administration hub that would suddenly find itself without anything to administrate. Tourist guides wouldn't understand anything.
And so on. The immediate consequences would be vast and destructive. People would wonder if they were insane, only be comforted by the fact that lots of other people seem insane too.
And then there would be the food problem. It would vary wildly from country to country. Many countries would face severe problems because they import much of their food. Others would have the smaller problem of suddenly having too much food.
We also export and import other products, such as oil and natural gas. Lots of power plants would stop working for lack of supplies. Oil pipelines would rupture at the border and start polluting. And nobody would know why!
In the longer term, many countries would find themselves without cars, or clothes, or medicines, or lots of other imported goods. And others would have enormous stockpiles of these things.
The Moon would crash into a force field and fall down Wait, that is not right. The Moon is above geostationary orbit. Instead a force field would crash into the Moon and force it away from the Earth. Depending on how the force field is anchored this could end badly for both the Moon and the Earth.
As a rule of thumb, smaller countries are more dependent on others than larger ones, so they would be harder hit. The US, Russia and China would probably do fairly well, at least compared to the worst disaster areas.
There are undoubtedly many exceptions to this general rule.
But all things pass. Enough people die that the country can feed itself, industries spring up to replace imports and so on. Eventually things will be fairly normal.
There would be vast militaries that would wonder why they exist, and would try to find some purpose in life. Many places would become military dictatorships. Others would devolve into civil wars.
Even this will pass. Civil wars will wither be won or end in a set of smaller countries. Militaries will downsize into ordinary government and police forces.
In the chaotic years, there will not be any advances in technologies or other things.
Afterwards, progress will be slower than today for two reasons. First is that the war industry is one of the forces behind new technology. There are also other forces, but they are weaker.
The second reason is that good ideas spread out and cause other good ideas. The more people in a community, the more ideas get invented and shared. Smaller countries might end up with no progress at all.
I think in Europe only France is capable of being self sufficient in food production. (Which of course don't mean they don't import anything).
with the wipeout you would need to clean the memory of foods, culture, language (my god, imagine the poor English), geography, science discoveries made thanks to travel etc.
So you would need to push countries to circa 7000 years ago? I think that's safe time where people we're doing ok without "Guten aben maine strange yellow people" or something like that.
BUT to answer your question: They would not. They would crumble and go back with technology to maybe XVII century. Maybeeee XIX if they're lucky.
