You can save posts as drafts before publishing them. In that case, you do not choose 'Post', but 'Save a draft.'
You can find your concept under Posts > Drafts:
Did you accidentally delete a message?
This can usually be fixed:
1. Go to Posts > Create new message.
2. Choose Restore.
Please note! This function only works when you use the Advanced edit method:
Save draft on behalf of someone else? In that case, the concept becomes invisible to you.