When people first start building a civilization most of their urban centers are going to be near water. This is important because that is the easiest way to move large volumes of goods. The big coastal cities are the first places that people gather when they finally move past the hand-to-mouth level of survival.
20,000 years ago the oceans of the world were 400 feet/122 meters lower than they are now. That changed the face of the world in vast ways. If a human civilization rose before or during that time their cities like ours would tend to be in the coastal areas.
There is some debate about exactly how abruptly things warmed up at the end of that ice age. However it happened, the devastation and destruction of ALL of the major coastal cities was total. If this happened at a time when the civilization was similar to what we had in say the 1500s their entire civilization may have totally collapsed.
The people of that time and place MAY have not been strictly or exactly home sapiens.. When the scattered survivors met the people coming out of Asia across the ice into North America they may have been totally wiped out and into extinction.
???? Who knows. This is all pure speculation....