With a light flap or soft thud — used for things closing or falling lightly (lighter than ばたっと); often written in katakana.
カーテンが風でパタッと閉まった。
The curtain closed with a light flap in the wind.
扉を静かに閉めたら、パタッと音がした。
When I closed the door quietly, it made a soft 'pat' sound.
Suddenly (stop, come to an end) — used when an action or state ends abruptly.
彼からの連絡がパタッと途絶えた。
His messages stopped all of a sudden.
会場の音楽がパタッと止まり、会場内が静かになった。
The music in the hall stopped abruptly, and it became quiet inside.