Email server on Galaxy S4 won't sync

A

AC Question

Recently in the last 2 months I keep getting a server error that says the email server will not sync. I have resigned in the last time and it worked fine, now when I try to sign in with my password it says Authentication failed. I have not changed my password. I did do a Force Stop on it this time, and don't know how to make sure that it is up and running again. Can anyone help me?