First thing I'd try is switching the iPad off and on again by holding down the power button at the top.
If that doesn't work delete your BT Internet Account from Settings>Mail. Then re add your email account again - messages are all up on the server so you won't loose anything.
One more thought, have you checked how much space your iPad has left in Settings>About? if you have no space left try deleting some unused apps or photos/videos etc.