Answer: the heart
When Anna and her friends reach Elsa's palace, Anna goes in to speak to her sister. But what starts as a friendly conversation, ends with Elsa getting agitated and striking Anna in the heart, with her powers, by accident. Even more upset by hurting her sister again, Elsa creates a giant snow monster which chases Anna, Kristoff, Sven, and Olaf away from her palace.
Although the consequences of being struck in the heart by Elsa do not effect Anna at first, her hair soon begins to turn whiter and she also becomes very cold. Realizing something is wrong, Kristoff takes Anna to his family, the trolls from the beginning of the movie. At first the trolls think that Anna is Kristoff's girlfriend and sing the song "Fixer Upper". Eventually, after they see that Anna is ill, the two are told by the Grand Pabbie that to thaw a frozen heart an act of true love must be performed. Believing this means a kiss from Hans, the group of friends quickly heads back to the castle.
Meanwhile, Prince Hans, who was left in charge of Arendelle by Anna when she left, has gone into the mountains with a group of men to find Anna and Elsa. They end up at Elsa's palace not long after the others have left. Hans and a couple of the men make it past the snow monster and capture Elsa. They bring her back to the castle and lock her up.