The woodland period is the period the American Indians were in when we discovered them. They used to have cities and functional civilization some time before our discovery of them, but somehow that collapsed and they entered a long dark ages, which is how we discovered them.
What do you think caused civilization to collapse for so long, and what kept it from re-igniting?
Variety of reasons. In general smallpox seems to be what hit most of the continent, and the collapse didn't really kick in until Columbus' landing. Precolumbian Mesoamerican civilizations collapsed pretty frequently; Teotihuacan having fallen when a nearby volcanic eruption blocked out the sun long enough to cause crop failure, and the Mayans having fallen (theoretically) after deforestation caused their ground water to dry up and, again, they succumb to crop failure. The Aztecs, who came after both of them, lived with a cyclical idea of all civilizations rising and then falling after a short time, and some feared they were approaching the end shortly before the Spanish arrived.
I'm no expert, I just saw some documentaries a few years ago, so take what I say with a grain of salt, and crush that grain into a powder.
CGP Grey had an interesting take on it, suggesting that domesticated animals were a huge pressure to create cities. North America severely lacked in domesticable animals.