be taken by surprise

To be happened upon when one is not ready, prepared, or on guard; to be shocked or startled by someone or something. I was taken completely by surprise when you showed up at my office this afternoon for lunch. I wish you would have called ahead! The enemy encampment was taken by surprise when our troops descended upon them just before daybreak. I never thought Andrew had any romantic interest in me, so I was taken by surprise when he asked me on a date.