Try not to fall into previous feelings and thoughts with her now. Take the card at face value: a birthday wish. She may have changed her ways, you can't know. It's up to you to decide if you are able to give her another chance. It might become a good friendship.

What to say? Thank her for remembering you, and how you often wondered if she did, and if she is willing to try and rekindle the friendship... Short and sweet?

Good wishes.