Try closing the Mail app via the multitasking bar and see if it works when you re-open the app :
iOS 6 and below : from the home screen (i.e. not with the Mail app 'open' on-screen) double-click the home button to bring up the taskbar, then press and hold any of the apps on the taskbar for a couple of seconds or so until they start shaking, then press the '-' in the top left of the Mail app to close it, and touch any part of the screen above the taskbar so as to stop the shaking and close the taskbar.
iOS 7 : doubleclick the home button to open the bar, then swipe or drag up the Mail app from there and off the top of the screen
If that doesn't fix it then try a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.