Hi, sailberg! Welcome to iPF!
Here are a few things you could try:
Reset your device: press and hold Home and Power button at the same time until the Apple logo appears, then release both. The iPad will start on it's own.
Force close the app:
iOS 6: double press Home button to reveal multitask bar, then tap and hold an app icon until they start to wiggle, delete apps there.
iOS 7: also double press Home button, now slide thumbnails (pictures of the apps) above the app icons towards the top of the screen.
Turn off Mail in Settings - Mails, Contacts, Calendars - the email account. Wait for a short time, maybe restart your device (turn off, turn on again), then enable Mail again.
Take a closer look at the setup: is it installed as POP or IMAP account?
Delete the account and reinstall it.