When that happened to me, I found I was working on an email that had been classified as a Draft. To convert a Draft to a "real" email, I had to choose "Send Again" from a Menu. It does not send it again, it just makes your Draft into a "live" email so you can work with it.
I hate the text on that Menu item, and the Human Interface Police really ought to arrest the guy who named it. I cringe when I use it because I have the nagging feeling it is going to do something awful.