See someone's text message but don't want to reply. If she asks next time, what's the good reason?

I think I saw someone's text message, but I don't want to reply. If she asks again next time, you can say that you are busy and want to reply to her when you are busy, but you forgot when you were busy, or your message was accidentally deleted by your family and you didn't see it. In this case, you won't feel embarrassed if the explanation is reasonable.