If you are having difficulties, it can often be simplest and most expedient to delete the email account from your iPad - then re-add the account. Assuming that you are using an IMAP mailbox, no mail messages will be lost - as all mail is stored on the mail server.
To delete the email account:
Settings > Mail > Accounts > [Your email account] > Delete Account
A forced-restart is recommended prior to re-adding the email account:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
After restarting the iPad, re-add your email account:
Add an email account to your iPhone, iPad, or iPod touch - Apple Support