You've not told us whether you're using an email client (such as Outlook Express, Windows Live Mail) or a web-based service (which you access via your web browser, such as Internet Explorer, Chrome or Firefox). It makes a difference!
However your reference to a 'Back' button suggests that you're using a web-based system. If you simply selected 'Delete' your mail should still be in a folder labelled 'Trash', 'Deleted' or something similar. But if you then deleted it from that folder it's probably gone forever (although a few email services might enable you to still retrieve it - we'd need to know who your email provider is in order to check).