An old email has vanished from my account after accessing it and viewing the attachments online.

I accessed an old email yesterday out of one of my folders. I viewed 2 of the 3 attachments contained in the email online. When I hit back to view the 3rd attachment I was brought back to the folder screen and the email dated early 2012 was gone. I have searched everywhere for this email, junk, deleted, POP, I trawled thought every folder and have used the search function extensively. If something similar has happened to you and you can help please let me know. I need access to this email!