I agree with the above response. "Why the long face?" sound much better to me if you are talking about somebody who looks sad. If you are talking about something shocking, you could say, "His jaw dropped" or "My jaw dropped" referring to shocking news that has taken you or somebody by surprise.

Why did your face fall is not used in English so no, it doesn't sound natural. We sometimes use 'Why the long face?' but it isn't very sympathetic and is normally used in jest. On this occasion you could always just ask what is wrong or 'what has upset you?' Hope that helps, Wade.

The expression is fine and I'll quote some examples from online dictionaries: Merriam-Webster: "Her face fell when I told her the bad news." Cambridge: "Her face fell when she heard he wasn't coming." This means there is a physical change in that person's face so that they look sad or disappointed, so it's not exactly the same as looking unhappy because there must be a change observed.