Problems with the Yahoo account again, I see.
Take a look at your settings (Mails, Contacts, Calendars). Open the account and see if the password is entered there. If it 's disappeared, enter it again. Maybe it will work afterwards.
What else you can do is delete the account on your iPad and reinstall it.
Also access Yahoo through a browser and take a look at your settings within the account there. Maybe your password needs verification, or you need to setup a new one.
Hope that helps.