Using an example from the Holy Land, of how knowledge can be lost. In the 5th century BCE, Nehemiah knew of the location of the royal necropolis, where King David was buried. His tomb was located in the City of David. Centuries later, the Jewish historian Josephus placed the tomb of David on Mt. Zion.
Knowledge was being lost before the Dark Ages. It is estimated that we have no more than 10% of the total literature produced by the ancient Greeks and Romans. It gets worse the further back in time we go.
And would we understand what we uncover, if we don't have something to compare it to? We would think that tombs of Middle East potentates would be elaborate. However, in the time of King David, the tombs were tunnels dug into the side of a hill. They didn't get more elaborate until centuries later. We know how to identity the tunnels as tombs because archaeologists have found other similar tunnels in the Middle East.