In your admin dashboard you can search for post rejected, click “Show” to see the details of the post (author + topic title + content). When you know the topic title you can enter it and restore the post rejected.
If you reject the first post (aka a topic) there is no way to restore it from the UI.
In the last case because you are a moderator, the easiest thing that you can do is open a PM with the user and send him the content of the rejected topic asking to create a new topic (that you will approve).