Like Odin, Loki, and Thor, Hela is a literal god, which means she can't be killed easily. In fact, Thor doesn't even really try to kill her himself. Instead, he resurrects Surtur, a fire demon determined to bring forth Ragnarok (the destruction of Asgard), and allows him to destroy the land of Asgard, with Hela still standing on it.
In no scene did the audience get a good look at Hela's dead body or at her in her moment of death. Yes, Asgard was reduced to space dust, but that doesn't mean Hela was too. Her sorcery far eclipsed Loki's skills, and he faked his own death right in front of his brother. That said, it is worth noting that the destruction of Asgard itself could very well be enough to kill Hela, even if we didn't see her physical body die. It's stated multiple times in the film that Hela draws her power from Asgard — the physical place, not the people. She needs to go to Asgard to regain her power and maintain it throughout the film. With Asgard now destroyed, so is the source of Hela's power.
Hela had an arc in which Odin killed her to save Thor's life, but brought her back to life "to restore the natural balance and death." So, Hela coming back from the dead would not be unheard of.