Currently in this world there are many societies living were other societies used to. Some of them were developed from ancient societies and others, due to a variety of processes and situations, weren’t.
There are different outcomes when a society collides with another one. You have to define if your current society is a descendant from the old one, or if is a new one.
If is a descendant, some aspects will remain, some will be erased, some will be adapted, and some will be introduced. Some buildings can still be standing, same with statues, or different art forms. Some rituals and traditions, holidays, festivities, carnivals, among others. There’s a chance that same food is still around, probably some recipes still remain, the same with clothes. Language is one of the cultural aspects that changes most through time. There’s a chance people can know and understand old words but the meaning will have changed.
The same with economical and political system, with gender and sexual identities, with social roles, with life expectancy and other demographic variations, with religion, with names, among others.
If your current society has nothing to do with the old one, there are two more outcomes for what happened there. On one hand, the old society perished due to something you have to come up with, and, on the other hand, the first traces of your current society erased the old one. Using the first outcome you still have some traces of the old life that took place there. Everything left there will have a new meaning, and interpretations and theories will be around until people is finally adapted to old remains. Using the second outcome will look like the paragraph above, but what happened to the people who lived there first? Where they killed? did they die due to diseases they weren’t adapted to? are their descendants still around? what happens to them? how did the cultural crash happened?
You can erase people from Earth but you can’t erase every aspect of their life.
Define what outcome you want for the society you are building. Remember there are different societies within countries and regions. Don’t forget that. You can also do some research on societies that developed from an old one, societies that erased another ones, societies that no one know what happened to them, yet, and societies that adapted to another one and now live under the same political borders.