Crestfallen means suddenly downcast after hoping for better. The face falls, and everybody sees it.
The image is a bird's crest dropping, since a raised crest signals confidence in a great many species and a lowered one does the opposite. That makes it one of the most physically precise words in English for disappointment. It describes a moment rather than a mood, so somebody looks crestfallen at the news rather than being crestfallen for a week. Deflated does the same job informally.
1adjectivecrestfallenDisappointed and dispirited after a hope has failed.
“He looked utterly crestfallen when the list went up.”
From crest + fallen, from the appearance of a horse with its crest (head) on its chest after defeat in a battle.
Etymology from Wiktionary, CC BY-SA