Screaming isn't going to help you now! The dementor pays no attention to your words and continues sucking out the poor girl's soul. There's only one way to stop this, and there's no time to lose!

There you go!

A weak flash of light erupts from the tip of your wand, but it dies out before it has any effect on the dementor. You'll have to try again and fast! Come on, take a deep breath and calm down. Focus. Think of a detailed thought or memory, something that makes you truly happy.