This is the 10579th most frequent Russian word.
внезапной
Sudden.
The word 'внезапной' is used in the instrumental case to describe the storm as the cause of the event.
Внезапной бурей был разрушен мост.
The bridge was destroyed by a sudden storm.
Here, 'внезапной' is used in the instrumental case to describe the sound that prompted her reaction.
Она реагировала на внезапной звук, обернувшись.
She reacted to the sudden sound by turning around.
In this sentence, 'внезапной' is used in the genitive case to indicate the attributes of the stop being referred to.