your face falls
One's expression suddenly looks disappointed or disheartened, especially after being very hopeful or optimistic. My face fell when I saw that the meal I'd spent all day preparing had burned in the oven. I could see their faces fall when I told them how much it would cost to process the application.
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your face ˈfallsyou suddenly look disappointed or upset: He was quite cheerful until we told him the price. Then his face fell.
Farlex Partner Idioms Dictionary © Farlex 2017