>>42476837 (OP)
sadness isn't a problem that needs to be fixed, it's a normal and necessary human emotion.
as much as I'd love to hug and tickle her until she's happy again, those solutions may be temporary, and only serve to distract from the root of whatever's causing her sadness
the best thing I can do is offer listen if she's willing to talk, and to be there for her so she doesn't have to be sad by herself