Try fully closing the Mail app and then restarting or resetting the iPad. To do (apologies if you know):
Fully Close an App: Double tap the Home button (or do a 4-finger swipe up) to reveal the card switcher. Find the Mail app "card" and swipe up on it to fully close it. Don't worry, it won't remove the app from your iPad, just from the iPad's resident memory.
Then...
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
If that doesn't help, try this last step...
Reset: Hold both the power and home buttons until the screen goes dark and the Apple logo then appears (the slide to unlock screen may or may not appear). Once the logo appears, release the buttons. After a time, your homescreen will appear. Please note that it may take some time for the Apple logo to appear or for your homescreen to appear (or both). This is normal. Eventually, the homescreen will appear.
After one or more of these steps, try the Mail app again and see how it works. Good luck and let us know how you get on.
Marilyn