Question: What is the Japanese expression for:

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> Sorry, I wasn't listening
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For example, for when you're asked a question and realize that you have no idea what it's about as you've been thinking about something else instead of listening to your collocutor.

Answer: 
I think you could say...  

●ごめん、聞いてなかった。 / ごめん、なんて？/ ごめん、もう一回。 to your friend/family.  

●すいません、聞いてませんでした。/ すいません、もう一回お願いします。 to your teacher / in class.  

●すみません/申し訳ございません、もう一度お願いします/もう一回お願いします/もう一度お願いできますか。 to your boss / in a business meeting.
If you want to be brutally honest and admit you were simply zoned out, you could say something like

> 
> 本当にすみませんでしたが、実は（無我）夢中でした。（もう一度説明してくれますか）　→　I'm so sorry, but honestly I was daydreaming/spaced out. (Could you repeat that explanation?)
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