Yes. Under the ordinary two-bail wicket law, the wicket is broken if even one bail is completely removed from the tops of the stumps. The second bail does not also need to fall. This describes when the wicket is broken; it does not, by itself, prove that the batter is out.
How one fallen bail changes the position
Hypothetical example: A legal delivery reaches the stumps. It removes the left bail completely, while the right bail remains in place. The wicket is broken because one bail has been entirely removed. The result is not automatically a dismissal: the relevant dismissal law and the circumstances of the play must still establish that the batter is out.
A bail that has merely moved, tilted, or vibrated but remains resting on the stumps does not meet the same threshold. However, if it has come completely away from the tops, it can satisfy the wicket-breaking requirement even when the other bail stays undisturbed. The official wording also recognises a bail falling and lodging between two stumps as completely removed.
Contrast: movement is not complete removal
Counterexample: The ball strikes a bail. It wobbles, lifts briefly, and settles back on the stumps without becoming completely detached. On those facts, the wicket is not broken merely because the bail moved. A clear disturbance is therefore different from the complete removal of one bail.
Our comparison uses ball contact. Other causes require checking the law’s separate conditions for breaking the wicket fairly.
What this rule does not decide
A useful incident note separates what moved, what caused the movement and any official dismissal decision. “Left bail removed, right bail retained” describes the example precisely without prematurely labelling the batter out.
This explanation concerns the ordinary wicket structure and does not cover play conducted without bails. Playing conditions for a particular competition may modify how an incident is administered. Read the relevant provision in the MCC Law on when the wicket is down.