In French, in this situation, i.e. you saw someone, didn't recognise them, *then* talked to them and recognised them *afterwards*, you will need Le Plus-que-Parfait, as explained by Laura. Le Passé Composé would be used to state a simple fact in the past, as in: "Je ne t'ai pas reconnu ce jour-là." / "Je ne t'ai pas reconnue quand tu es ... More @Wikipedia
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