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This usually happens if the Mail application itself is corrupted. Sometimes it happens if there is any issue with the mailboxes/accounts. If it is the Mail application, then we have to restore your iPad from iTunes on your computer. Before doing that, we can do some troubleshooting steps on the iPad.
First reset the iPad. To reset, press and hold down the Power button and Home button at the same time until it shows the Apple logo (ignore slide to turn off message). When you see the Apple logo, release the buttons and let it boot. Check Mail. Does it work ?
If that didn't work, then it is possible that one of the email account is causing this issue. We need to find out which one is corrupted. Go to Settings > Mail, Contacts, Calendars and you will see all the emails accounts there. Delete one email account and check Mail. If it still having issue, then delete all email accounts. Then add the email accounts one by one and check Mail after loading each one.
Please let me know if it still not working, so I can send you the instructions to restore the iPad from iTunes on your computer without losing your files.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.