You could also improve your chances of getting good advice by letting us know what kind of service you are using.
In the case of GMail, MobileMe, and Yahoo!, it is often easier to simply delete and re-add the account than to try and figure out what went wrong. That would probably work with other account types too, but since I don't use them I'm less willing to sound confident about doing things this way.
If you also sync Calendars or Contacts with this account, be sure to allow it to remove them from the iPad when deleting, or you may get duplicates when adding the account again. If your Contacts and/or Calendars are local (not synced elsewhere), do not let it remove them or overwrite them, or they will be gone, gone, gone. . .