Hi There. I am Bing and I look forward to helping you with your question today.
Basically, you mean you are looking for an email you sent last night and now you can't find it anywhere including the Trash folder. Am I correct?
Moreover, please advise what email service you are referring to, such as Gmail, Yahoo, AOL or Outlook.com? Also please advise which email program you used to write that missing email? Or you wrote it directly on web using Firefox?