Somebody’s heart sinks
Definition 1 of 1
Pro Tip 1/3Emotional Impact
Used to emphasize strong feelings of disappointment or discouragement. 
Her heart sank as the storm destroyed her new garden.
Pro Tip 2/3Sudden Change
Reflects an immediate drop in emotion or spirit, often unexpected. 
His heart sank when he realized he left his tickets at home.
Pro Tip 3/3Not Literal
It's a metaphorical expression; the heart doesn't actually move downward. 
Her heart sank, not with illness, but from the weight of her worries.