If neither of the links Lin posted are the one you're looking for, try this:
Click on the little person icon - it's in the blue bar on the left of the screen, and will take you to your profile.
In the "Actions" box, you'll see a link called "View posts made by louise". That will show all the discussions you've participated in.
If you were only reading and not commenting, I'm not sure how to track down a thread without knowing the title. In the future (or when you find this one), leave a comment, or click the thumb or the star anywhere in the thread, and you'll be able to access the thread from your profile anytime. There's also a link that says "Watch" at the top of every thread, and if you click it, you'll get a notification on the front page every time there's a new post.
