Something sent cannot be unsent. If it was in your "Sent Mail" label, and not Drafts or Outbox, or you have the delayed send lab turned on, then it had already been sent them to them. GMail has no way to notify you a message has been sent, other than to keep it with the sent mail label.
If you deleted it, it should be in your Trash label, unless you meant that you further deleted it from there as well. If so, I have a hard time with your wording of accidentally deleting it, cause you had to click delete at least twice... hard to do by accident. But regardless of that, if you'd already clicked on "Send" and it's been more than 30 seconds... it was already gone and across the internets towards them.