Arendelle was completely frozen as a result of Elsa's winter.
Elsa's winter was a period of abnormal weather characterized by the freezing of summer in Arendelle. During this period, temperatures plummeted, and ice and snow covered the landscape. It was triggered by Elsa's flight from Arendelle when she completely froze the fjord. In the hopes that Elsa could reverse her magic, Anna set out to retrieve her sister, and summer was ultimately restored through the Great Thaw.
History
Prelude
After being aggravated by Anna, Elsa exposed her powers to the kingdom and decided to flee. However, as she raced across the fjord, Elsa inadvertently caused the water to freeze; after the queen fled into the forest, snow began to fall, worrying the citizens of Arendelle. Hoping to bring back summer, Anna set out in search of her sister, leaving Prince Hans in charge during her absence.
Elsa fled to the North Mountain, where she constructed an ice palace, intent on living alone to freely use her powers. However, unbeknownst to her, snow continued to fall on the kingdom, leading to Arendelle becoming completely frozen.
Journey to restore summer
Navigation through the forest was made difficult through the heavy snowfall.
The harsh weather conditions made Anna's journey into the forest difficult, but she was able to recruit the aid of Kristoff and his reindeer, Sven, at Wandering Oaken's Trading Post and Sauna. However, after being forced to abandon Kristoff's sled while escaping from wolves, the trio was forced to continue on foot, during which they encountered Olaf, who helped lead the group to Elsa's palace. Ultimately, Anna was unable to convince Elsa to return to Arendelle, only serving to alarm her sister upon revealing that the kingdom was affected by "eternal winter".
Worsening conditions
In the meantime, the citizens of Arendelle were having a difficult time trying to stay warm. Hans had opened up the castle for food and shelter; however, tension among the villagers remained high, with a pair of villagers even bickering over the placement of wood. Morale worsened when Kjekk returned without Anna, but Hans gathered a group of volunteers to find her. Though he was unsuccessful in his search for the princess, Hans was able to capture Elsa, who was bound and placed in the dungeon.
But Elsa's presence in Arendelle only led to escalation of weather conditions; despite being trapped in the dungeon, Elsa's emotions caused the wind to pick up and temperatures to plummet. Upon her escape from imprisonment, a blizzard had begun to ravage the castle area, making navigation on the fjord nearly impossible. Nonetheless, Hans attempted to track Elsa through the turbulent conditions and execute her under the pretense that she had killed Anna. However, his plan was unsuccessful, as Anna was able to save her sister from the killing blow. After the two sisters reconciled, Elsa was able to use love as a means to thaw Arendelle.