If you suddenly is unable to log in onto your Gmail account, or your email client keeps prompting you for username and password due to invalid credential or Web Login Required, most likely your account is locked. In most cases, you are still able to log in directly with Gmail Web Login.
In order to unlock your account, use Google accounts' UnlockCaptcha. Everything should be reset in a second.
Trouble Login onto Gmail: https://mail.google.com/support/bin/answer.py?answer=78754