Once-happy is an expression used to describe something or someone who was once filled with joy or happiness but has lost that feeling over time. There are several synonyms that can be used to describe this state of being, such as melancholic, despondent, miserable, sorrowful, crestfallen or disheartened. These words give a more nuanced reflection of the loss of happiness, and can be used to capture the depth of emotion that might accompany that loss. Each synonym has its own unique connotation, so the appropriate word choice will depend on the context in which it is being used.