"Hey [name], I'm Cat, remember me? I haven't seen you in X years/since X grade, how are you doing?". Possible topics to bring up for silences: where was he/she, how does it feel to be back, how does he/she feel about how your local zone changed. When you feel like it, end with "Ok, it was nice seeing you, I'm glad you are back, see you around", or even ask them to hang out sometimes.
Keep it casual, maybe short, it's perfectly social and polite, they will not be freaked out (unless the last time they saw you you chased them with an axe, or they have a perfectly good reason you gave them to be scared of you, if that's the case, avoid them).