Has the mail been working but stopped?
Try totally shutting iPad down by holding bottom button until red slider comes at top of screen and slide it.
If iPad 4 or new one press home button once until all open apps come up and slide the unwanted apps to top of screen, including mail one. This shuts the app.
If that doesn't work the try "reset all settings" it's in settings, general rest. This does not erase anything.
If all else fails. Then try setting, mail and delete email account and reinstall it but only if you can remember passwords etc!