How do I start talking to a guy again after I rejected him?

Do you have his email address? If so, I would write to him and tell him that you saw him at the party and felt he was uncomfortable with you. You can say that you miss him and would like to resume the friendship but if you still clearly feel that you don’t want a relationship with him, you need to say that too. Ask him to respond if he cares to.

You need to give him the room to decide whether he wants a friendship on your terms again or not which is why I suggest writing to him. That gives him the space to decide. And whichever way he decides, you have to respect that it is what he wants.

He clearly moved on.
That’s why he didn’t notice you at the party.
Do him a favor and let it be.

Are you friends on FB? You can PM (or DM or whatever you call it) him and say hi. Be prepared for him to not be interested and in that case, let it drop.

Simply contact him through FB, friends or whatever.
If he isn’t interested, so what? There are far more painful things that can happen.

Be cool. Be nonchalant. Be natural. Act like nothing has ever gone on between you two in the past. Say “Hi.” Talk about the weather. Talk about COVID-19. Ask him if he’d like to get a cup of coffee.

You’ve changed in two years. So has he. Be sure to allow for that in whatever you do next.

Just put your lips together & blow.
Wait, that can’t be right…

I think just reach out and tell him you weren’t ready for a relationship, but you appreciated having him in your life and you miss him.

Are you looking for friendship or possible relationship? If it’s just friendship, then that’s not fair to him (assuming he’s still interested in you). You’d put him in an awkward place every time you talked.

If you are interested in something more than friendship, it wouldn’t be wrong to reach out.