Early in Thor: Ragnarok, the God of Thunder and Loki are stood beside their father, Odin, who explains that he is dying and with his end, comes a great threat, Hela. Odin had been able to keep Hela imprisoned for centuries, but his death would lower the enchantment placed on her prison, thus freeing her. After her arrival, Thor throws Mjolnir at her, only for the Goddess of Death to catch and shatter it into pieces. Since the film's release, audiences have been flabbergasted by how she was able to achieve such a feat of strength. Fortunately, a new Reddit theory might have solved the conundrum.
Reddit user TheOrdinaryCritic posted a thread explaining his theory about what happens after Odin's death. At the time of his passing, the All-Father stated that his magic would no longer be able to hold Hela, which could mean that his death lowers or removes all of his enchantments, including the one placed on Mjolnir.
During the first Thor film, Odin enchanted Mjolnir by stating "Whosoever holds this hammer, if they be worthy, shall possess the power of Thor." Prior to this point, Mjolnir was already a powerful weapon, capable of concentrating Thor's power and lightning, but the enchantment made it so that the hammer had an infinite mass that could only be altered when held by someone who is worthy. However, with Odin's magic dispersed from the universe after his death, anyone strong enough to wield the hammer would be able to.
While the theory seems to hold up on paper, the comments go on to question it in relation to a possible "dead man's spell." During Avengers: Infinity War, Doctor Strange tells Maw that the dead man's spell placed on the Eye of Agamoto is impossible to break, meaning it's possible the same could be applied to Mjolnir. However, the two cases are different because the magic of Earth-based sorcerers and those of Asgardian deities comes from a different source. As one commenter points out, Strange's magic alters the fabric of reality using spells and incantations, while Odin's magic is purer.
So, without an actual explanation as to how Odin's magic works, it's hard to say whether or not his enchantments would continue to have an effect after his death. The only firm in-universe evidence that seems to back the theory up is that when Hela caught the hammer, Thor and Loki seemed shocked, even after their father had explained that the power keeping her trapped would dissipate upon his death.
Regardless, as fun as this theory is, it just doesn't appear to be a sound explanation when looking at what has been explored about Asgardian magic in the Marvel Cinematic Universe. It's more likely that Hela's pure strength and previous attachment to the hammer from her time conquering worlds with her father had more to do with her being able to lift it in Thor: Ragnarok than anything else.