Make sure you are typing in your correct email and password. Sometimes we make mistakes, especially with passwords since they are case-sensitive. If you typed a capital letter when you first created your password you will need to do the same when trying to logon again. Try different versions of your password. As a last resort, request a new password from your service provider or forget about that email address all together.
Sometimes email services are temporarily down, so wait a while then try to reconnect later.

