How to get over someone even if the two of didn't have a relationship?

You block them out and distract yourself till you don’t even think about them anymore. Do things you like and focus on what you want for a change.

First of all, let yourself feel whatever you are feeling. Don't think that you aren't entitled to feel upset or heartbroken just because you weren't together. Talk to your friends, try to go out and spend time with them or do things you enjoy. Avoid that person so that you won't hurt more later on. Treat it almost like an actual breakup because, as far as your heart is concerned, that's often exactly what it feels like.