When you change your password to an email service, sometimes your iPhone will keep the old password and not allow you to retrieve your email. To correct this, you will need to go to the Settings app. Once there tap on the “Mail, Calendars, and Contacts” section. Tap the account that needs the password corrected. Then update the password below and hit done in the upper right.
Once that has been completed your emails should start rolling back in like normal.