There is a huge sun theme in Tangled and this song fits perfectly to what happens when Rapunzel sees Eugene (Flynn Rider) die. She had finally had sun in her life, but he was taken away and she thought her own sundrop magic was gone, so there once was the sun's magic in her and it was now no more.

Quotes that really work with this scenario:

"Once there was the sun"
"Now there's no more sun"
"Winter has killed everything"

^Changing my vote because you made some good points