Look at her with big sad eyes and say "I really need a hug." (Edit) Ooops! I thought she told YOU to cheer up! In this case, ask her if SHE needs a hug. Tell her a self-deprecating funny story, something you did so dumb you'd never tell anybody else. That'll show her that she's very special and nothing cheers up a girl more than that (except maybe diamonds! haha)
When a girl tells you to cheer her up, you can tell her a joke or imitate a cartoon or clown. You can also take to a place that she like her favorite club and make her dance with you. However, be careful with what you say, since a girl in an emotional state if very fragile and can be offended by the smallest words. See www.wikihow.com/Cheer-up-a-Woman for tips on cheering a girl up.