、。
Stay down, or you'll get shot.
Literal
Prone [condition-のまま] stay, otherwise shot-will-be.
伏せたまま uses ~たまま (remaining in the state of having done): 'stay lying face-down.' さもないと means 'otherwise/if you don't.' 撃たれるぞ is passive (get shot) + masculine emphasis ぞ. Urgent, tense.