Check your mail account settings to make sure they are entered correctly. Chances are they are ok since your mail worked prevusly, correct?
First thing to try is a reboot of your device. Press and hold the Home and Sleep buttons simultaneously ignoring the red slider until the Apple logo appears. Let go of the buttons and let the device restart. See if that fixes your problem.
If that doesn't help contact your Internet service provider to see if their email service is experiencing problems.
iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps
Try this: Delete the account in Mail and then set it up again.