Check the following:

• Have you entered the correct username and password? Passwords are case sensitive, so make sure that you enter the password exactly as it was written.
• Are you a member of the domain?
• Has the user link expired? If so, contact your system administrator for a new one.
• Are you enrolled on a course and is it active (not expired)? For admins, this is in the user settings > courses.

If none of these are the cause of the issue or you get an error message, contact Support for assistance.