You are not missing anything. That is a little "bug" in the mail app that I experience all the time with a saved draft.
Quit the mail app completely and then start mail again. Go to the home screen first by tapping the home button. Double tap the home button and the task bar will appear with all of your recent/open apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar.
Launch mail again and you should be able to work with the draft again.