If you know your login and password it is possible that your Internet browser is blocking the window needed to verify your login and password.
To resolve this issue:
Specific instructions vary depening on your version of Internet Explorer, Firefox, Chrome, Safari, Opera, etc. Refer to your browser's documentation for details on how to allow sites to bypass the pop-up blocker.
You may also have add-on toolbars such as the Google Toolbar or security software such as Norton or McAfee that block pop-ups Pop-ups may also need to be allowed from these services.