Believe it or not, there was a brief period where we all thought Eleven died in season one. During the final showdown at Hawkins Middle School, Eleven confronts the Demogorgon one final time to protect her friends. Weak from all that transpires up to that point, it seems like a losing battle. Fortunately, she succeeds in killing the Demogorgon, disintegrating it with her abilities. As the Demogorgon fades away, though, so does she, leaving her friends and the audience to wonder if stretching her abilities that far actually killed her.
Eleven's fakeout death wasn't drawn out terribly long, as we quickly find out that she's been transported to the Upside Down, still very much alive. We even see glimpses of Hopper leaving food out for her because he knows that she's still alive somewhere.
