In case you don't have Mail setup on your iPad specifically, Apple has posted some instructions which may help:

Extra Apple Support instructions for a variety of functionality on the iPhone/iPad can be found here :

Please note that in some cases mail can become stuck in the outbox so you may also need to initiate the Send again. If you goto the Outbox and tap the message and then tap the Subject field/line (as if you were to Edit it) then tap the Send button again hopefully this should send the email (provided the correct password settings have all been set up etc, as noted above).