Why would she stop eating?

Why would an apparently normal 19 year old girl suddenly stop eating? She hasn't eaten for 5 days now I'm very worried about her. She acts happy and cheerful, but its a front. She used to eat a good amount of food, but junk food, the wrong sorts of things. Now she won't even eat chocolate, which was her favourite junk food.
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Thanks for the replies. To be more clear, she isn't eating anything at the moment as far as we can tell. Ill try offering her healthy food. See how that goes.
Why would she stop eating?
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