You can try quitting mail completely - turn Mail off in that account - restart the iPad - turn mail on again - and then see if it will send emails.
Go to the home screen first by tapping the home button. Double tap the home button and the task bar will appear with all of your recent/open apps displayed at the bottom. Tap and hold down on the mail icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner to close the app. Tap the home button. when you are done.
Go to Settings>Mail, Contacts, Calendars>Your email account>Mail>Off
Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
Go back to Settings>Mail, Contacts, Calendars>Your email account>Mail>On
One thing that we tend to forget is the possibility that there are problems with your email provider - it has happened to me before - and not all that long ago in fact.