Societal collapse is when a group of people or a society stops functioning in the way it used to. This could happen due to a natural disaster or it could be caused by something bad people do, like starting a war. When this happens, a lot of scary or bad things can happen, like people not having enough food, not having places to live, crime and violence, and people feeling really sad and scared. It's really important that people work together to make sure things like this don't happen.