iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps
Try a Reset - iPad How-Tos http://ipod.about.com/lr/ipad_how-tos/903396/1/
Or this - Delete the account in Mail and then set it up again.
If your account is Yahoo, give it a day or so. Yahoo has been upgrading their system, causing persistently random outages for people. I've been unable to get it via the mail app, but can thorough the web interface, or this morning I can't get it via firefox on my laptop, but the ipad interface works. So if you happen to be using Yahoo, just give it time and find a work around because the issues are on their end.