2 email accounts, one is not showing up

I have 2 email accounts set up for cox.net.  I recently upgraded my Mac operating system to Sierra, and when I did one of the email accounts is no longer receiving emails, or showing up as an option when I select "Accounts."  When I tried to add the email account that is no longer showing up, I received a message stating that the email account already exists!  The email for both accounts DOES show up when I got to Cox Webmail.  Help!