Username or Password Incorrect

When trying to load new emails on my mail app it says "Cannot Get Mail The username or password is incorrect". It has been working fine since I got it 4 months ago until today. I tested my username and password on my mail account online to see if it had been hacked and it works just fine, so it is correct. I also tried re-entering my email address and password in the mail settings, but it still says they are incorrect. Can anyone help me, as this is very inconvenient?