I'd do a force-quit of the Mail app, immediately followed by a restart of the iPhone. That can deal with a fair amount of unexpected behaviors (on the assumption that this is just a temporary glitch).
Some forms of unexpected behavior are due to fundamental flaws - a true bug in an app or OS. Others may be due to one or more unusual occurrences that together cause something to misbehave, but are so rare that identifying them and rewriting code to prevent them may not be worth the effort. So generally, the first step with things like this is to wipe the slate clean (force-close/restart) and see if they return.