Mismatch

IM using my iPad mini and for some reason, all of a sudden, I cannot access my emails. The message comes up that says there is a mismatch between username and password. I hit the key to take me to settings and it asks for my password. I get the same error. It's basically telling me my password does not match the email username. It's like the username never existed. I've tried several times to log in and it won't give me new email. It also won't let me send emails. I can still see and pull up all of my old ones but nothing new. Please help. Frustrated!