Try quitting the Mail app completely. Double press the Home button to open the multi-task bar, press and hold any app in the multi-task bar until they all start to jiggle, then touch the top left hand corner of the Mail app to close it completely. It only closes it - it doesn't delete it.
Then try again. If that doesn't work, try powering the iPad down completely by pressing the Power button until the slider control appears. Accept the offer to power down and then, when the screen is completely blank, press the Power button again. A white Apple logo will appear and the iPad will power up - it takes a while. Then try again.
Tim