Eleven seemingly sacrificed herself to destroy the Demogorgon.
At the end of Season 1 of “Stranger Things,” a climactic confrontation takes place at Hawkins Middle School where Eleven and the other main characters face off against the Demogorgon, the monster from the Upside Down. Eleven, using her psychokinetic abilities, manages to overpower the creature. She holds it against a classroom wall, and as she does so, both she and the monster disintegrate into a cloud of dust and vanish, leaving her friends and viewers to believe that she sacrificed herself to save them and the town of Hawkins.
However, the season finale leaves some hints that Eleven’s story isn’t over. In a scene with Police Chief Jim Hopper, we see him leaving a box of Eleven’s favorite Eggo waffles in a concealed box in the woods, suggesting he believes or knows that Eleven is still alive. This sets the stage for her return in the subsequent seasons, where it is revealed what truly happened to her after the events of the Season 1 finale.