Click on your "mail" folder. Then click on "ignore list." From there you can unignore anyone on the list.
It is kind of easy to ignore someone by accident too. I have done it. I don't even believe in the ignore buttont, as I think that if you want to ignore someone, you just don't click on the post and open it. No need for a button. However when you are reading and replying to posts on a smart phone, it is easy to hit "ignore user" instead of "reply" or "report this post." They are all awfully close together on a phone screen So I have accidentally done it, and that is how I figured out how to undo it