Mail

A few ideas:

1. Log on to your mail provider using a web browser. Are your credentials accepted and can you view you emails?
2. If yes, delete this email account from your phone, restart your phone and recreate the email account
3. If not, please contact your email provider

Do you have this issue with one specific eMail provider or all of the ones you use? Do you access your emails via the Apple Mail app on your phone? What have you tried, so far yourself, to resolve this so that we don't try offering potential solutions that may not have already worked for you?