If you leave the forums and can't find your thread when you get back, you can see a list of all the threads you've been involved in on your profile page. To view your profile click on the "View your profile" link in the upper right of the header at the top of every forum page.
Note: The "View your profile" link is only available after you log in. You might want to login now, if you haven't already.
If you know you posted but you can't find it, it may have gotten caught by our over-ambitious spam filter. If that happens, don't panic. One of the volunteers will get around to finding it and clearing it, usually within an hour. You can get caught as spam if you post too many times in succession, or if your posts are all the same (yep, it can check that). It happens to pretty much everyone at least once. Don't worry about it too much.