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.