Probably nothing. It really depends on the situation. I have seen people from my past in passing (but they didn't see me) and I didn't say anything. I have spoken to a couple of people from 25-30 years ago, but it was mostly just quick pleasantries. I am not the person I was back then so I don't expect they are either.
If I run across the guy who molested me, I'm not sure how I would react. I met him several years ago at a funeral and we were friendly, but that was before I realized that what he did was wrong (and not my fault). I work within a kilometre of his house and I used to fantasize about confronting him, but even those fantasies turned out badly. There is no changing what happened and, for the most part, I have put it behind me. A little bit will always be with me, but it also made me who I am today and I'm reasonably happy with who I am now. I think if I bumped into him, I would be coldly civil. There is no point in giving him the power of knowing how he affected me those many years ago.