If you are using Internet Explorer as your internet browser you may run into an issue that has to do with how Windows handles saved passwords. There is information available online regarding how to clear saved passwords, which should resolve this issue, but it will likely occur again in the future.
We recommend downloading and using a modern browser for security, stability and user interface continuity reasons. You can read more about the browsers we recommend and why in the article: Browser Support: What & Why
If you continue to have issues related to logging in after you have confirmed the credentials you are using are correct, please submit a support ticket.