If you used the Save as Draft feature for a thread you started in a discussion board, you will use the Filter by tool under the discussion to locate your draft.
First, locate the discussion either through the Table of Contents or School Tools > Discussions.
Below the discussion board, locate the Filter by option, which may be directly underneath the Start a New Thread button. The default should be set to All Threads (see image below).
Click the down arrow to the right of All Threads and check the box in front of Drafts.
You should now see your draft and be able to click add text to the message area. To publish it for others to see, click the Post button.