There are a few possibilities. First, it could simply be in a different place than you thought you put it. Or second, I could've moved it to a more suitable location. (This is especially true if you posted in the Random Knitting-Related Stuff section, which I clean out all the time.)
Either way, don't panic. There's an easy way to find your post. Simply go to the forum homepage and click on your name in the Active Users box along the top of the page.
This will take you to your profile, and you'll see a header marked "Recent Topics." Bingo! Your most recent posts will be listed there.
If it wasn't so recent, simply click on the "view all non-archived posts by this member" link to see all your posts from approximately the last three months.
You can also do the same search on any other forum member by clicking on their name in the Active Users box.