Your best bet is to fully close the Mail app, then to re-open it. To fully close it (apologies if you know): Double tap the Home button (or do a 4-finger swipe up) to reveal a row of icons in the multi-tasking bar. Press and hold an icon until they all start to wiggle and you see a red circle with a minus sign ("-") in the top left. Press the minus sign to fully close the app. Don't worry, it won't remove the app from your iPad, just from the iPad's resident memory.
See if that helped? If yes, yay! If not, holler back and we'll try something else.
Marilyn