Welcome to Android Central! Which phone, which email app, and which email provider? Can you still log into your email on the web (if there's a webmail interface for it)? Does the email provider have 2-step authentication?
If it's a server error, someone made a change to the server and the code on the server is bad. Contact the email provider (which may be difficult until you create a new free email account somewhere.)
If it's a password error, download a different email app, then create the account in that app (as an existing account). Make sure you're getting the password right - case counts. See if that works.
If nothing B. Diddy or I said helps you, join the Android Central community and add a post to this thread (you'll see a box if you scroll down once you've joined), and we'll try to help you further.