Hello. If you could elaborate and let us know :
Device 
Email app
Email service/account (Gmail /Yahoo....) 
Did it work on that device before 
Can you get your email on a pc or other device. 
Is this devices data working otherwise 
Have you changed settings 
Have you just upgraded to Lollipop? 
You would need to register a Username and password to post again in this thread so someone can help. Thanks.