My Samsung Galaxy S4 is showing sign in failed. What does this mean and how do I fix it?

[AC Question]

I am not receiving emails on my phone. Showing sign in failed.

 

[B. Diddy]

Welcome to Android Central! Sign-in where? Are you using the stock Email app, the Gmail app, or something else? Which email account isn't receiving? Could it be an iCloud ".me" account?