Hi Linda,
There are a few ways you can try, the first is pretty easy. Just go to the forum that should contain the post. Somewhere at the top of the page, depending on your theme, there should be a small text box with a, Search forums, button next to it. Type one or two words, that you know were in the post, into the text box, then click the button. Cross your fingers and hope it turns up what you are looking for.
If simple search doesn't find it, then after that first attempt, somewhere near the top of the page, you should see a link for, Advanced search... Click the link, then try some of the possibilities. Simplest would be to look for the author, if you know the correct name to look for.
If neither of those two work, most likely the post is gone, but you can go look in the database using phpMyAdmin or Adminer. The correct table is mdl_forum_posts and the fields you would want to search are subject and message. But, I do know from previous experiments, that I just repeated to make sure, if the forum post was deleted, then it is no longer in that table and you will not be able to recover it unless you have a backup of your whole database from before the post was deleted.