At the end of the movie, Eleven sacrificed himself to kill the monster Demogorgon, saving Will back to the real world. But at the last minute of the gathering, viewers knew that Upside Down was still not done with Will.
A year has passed, we don’t know how Eleven’s Upside Down life is, how she returns to the human world. But the viewer knows for sure that she is alive. Evidence is the end of the first season, police chief Jim Hopper has put pancakes in a black car. Hopper brought food to Eleven. She is hiding something and must rely on the police chief’s help.