Hello. My name is, Mike. You said that you composed an email message in FB, Unfortunately in FB they don't cache (temporarily save) information. Once you hit dismiss, you clear all temporary unsent information. The only way to see if it is still there is to click on your messages icon and see if anything appears that hasn't been sent. Once gone, there is no way to retrieve it. It is not like a normal email account where if you forget to send an email it automatically saves it in your "Drafts" folder.