It would have 'melted your heart' [=it would have filled you with compassion, sympathy, etc.] to see her lying in that hospital bed.
Her anger 'melted away' when she saw that he was truly sorry.
He seemed to 'melt into' thin air. [=he seemed to disappear]
(..) So, where's Hans? What happened to your kiss? I was wrong about him. It wasn't true love. Huh. But we ran all the way here? Please Olaf, you can't stay here, you'll melt. I am not leaving here until we find ...some other act of true love to save you. Do you happen to have any ideas? (..)2013 Frozen