|Need some more information...|
When's the last time you were able to log-in successfully?
Are you logging in via the website, or are you using Outlook (or another Email application)?
Do you get an error when you try to log in? What is the error?
If it's a "Wrong/Bad/Incorrect Password" type error:
1) triple-double-check that your email address is typed properly
2) make sure your keyboard's CAPS LOCK is off
3) if you're still getting a Bad Password error, follow the Password Reset procedure.
-- (Click the blue 'Forgot Password' link under the Password box.)