The question caught her completely off-guard and she did not know how to reply.

这个问题令她猝不及防，不知道怎么回答。

《柯林斯英汉双解大词典》

But he completely caught me off my guard; he strode into my life during that season and set me gently but firmly on a new path.

但是他还是在我猝不及防的时候出现了。他在那个时段进入我的生活，让我重新踏上一条新的道路。

youdao

Girl you caught me completely off guard.

女孩，你让我完全丧失了警惕。

youdao

Ms. MATAMBANADZO: We were caught completely off-guard by the attack at the offices, and to this day we are quite startled about why it happened.

NPR: Zimbabwe Government Goes After Media