all at once

all at once  {adv. phr.}

1. At the same time; together.

The teacher told the children to talk one at a time; if they all talked at one time, she could not understand them.

Bill can play the piano, sing, and lead his orchestra all at once.

2. or all of a sudden

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.

Compare: AT ONCE.