Hi there, not sure if you have tried the immediate fixes when apps, such as your mail app misbehaves...but worth a try here:
Force the app to close down by pushing the home button twice to bring up your multi-task bar. This will show all the apps that you have visited. By long pressing the app in question, it will begin to jiggle-press the red circle with the negative sign on the app. It will only quit the app and not delete. Than try to re-open the app and test out.
If that doesn't work, restart the iPad by pressing the power switch down to power off.
Third is to do a soft reset by holding the power switch and home button together until you see the apple logo. Hope that helps!