all at once
At the same time; together.
The teacher told the children to talk one at a time; if they all talked at once, she could not understand them.
Bill can play the piano, sing, and lead his orchestra all at once.
Without warning; abruptly; suddenly; unexpectedly.
All at once we heard a shot and the soldier fell to the ground.
All of a sudden the ship struck a rock.
All at once Millie got up and left the house without any explanation.
at once
Without delay; right now or right then; immediately.
Put a burning match next to a piece of paper and it will begin burning at once.
Mother called the children to lunch, and Paul came at once, but Brenda stayed in the sand pile a little longer.
Stop playing that loud music at once!