Marvel Studios dropped the first official "Thor: Ragnarok" trailer recently, which gave fans plenty of ideas to surmise about the film's plot. The preview reveals the destruction of Asgard and that Hela is the main villain Thor has to defeat.
The highlight in the "Thor: Ragnarok" trailer is the destruction of Asgard following the appearance of Hela (Cate Blanchett). It is apparent from the preview that Hela has a built up anger. After all, in the comics she spent millennia trapped, unable to quench her thirst to create destruction.
Unfortunately, Asgard suffers the brunt of her anger in "Thor: Ragnarok" as revealed in the trailer. She takes over Thor's planet and proclaims it dead. According to Gizmodo, this is where the "Ragnarok" part of the movie matches with the comics. The death of Asgard harks to the death of the Asgardian gods.
The fate of Asgard lies on its hero, Thor. However, the Norse god seems incapacitated to protect his home. Thor seems no match to Hela, whose clearly stronger and powerful than he is. She manages to crush the Mjolnir with ease.
However, in the comics, the Mjolnir has been destroyed several times, according to The Hollywood Reporter. So it is likely that the hammer pieces itself back together at some point in "Thor: Ragnarok" when Thor returns to save Asgard.
Speaking of Thor, the Norse god is unable to protect his home early in the movie. The trailer for "Thor: Ragnarok" shows him transported to Sakaar, where he is pitted against Hulk in a gladiatorial fight. He also loses his trademark long hair, which happened in the comics.
Regardless, the presence of Valkyrie suggests that she helps Thor escape Sakaar, defeat Hela and save Asgard. The "Thor: Ragnarok" trailer hints that she and Hela had a bad history, wherein during a fight the villain sends her in Sakaar. This would explain why she is hanging around in the planet.