First of all, check your Trash folder. Deleting a message in most instances simply moves it to Trash. If there are a lot messages in your Trash folder, you can search for the message the message you're looking for. You can also sort "by date of deletion" to quickly find a message you just deleted, even if the message itself is quite old.
If you find something your want to keep in your Trash, use the "Undelete" button to restore the messages in the conversation to their previous folder (before they were deleted to Trash).
If the message has been permanently deleted from Trash, you can may still be able to restore it from our backup. We keep backups of deleted email for one week. After that, it's gone forever.
To restore from Backup, go to the Settings → Restore from Backup screen. Read the the notes, and click the button to begin the restore when you're ready. Depending on the size of your account, the restore might take a few minutes or many hours. We will email you when it is complete.
Be sure to carefully read the email that is sent to you. The restored emails are placed into temporary folders that are deleted after a week. You must copy any email you want to keep from the temporary folder(s) to a standard folder.
Note: Message flags are not preserved for restored messages. This means that the restored messages will all appear unread.