There could be many reasons but the most probable is you have simply forgotton your password, are trying the wrong one, or are entering it incorrectly. Some other things to think about include:
Does your username or password contain a mixture of upper and lower case letters? It should be entered exactly.
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How do I gain access to a course?
Locate or search for the desired course (you can click 'All courses...' in the 'My courses' block) and click on the course name. If your teacher has given you an enrollment key, enter it when prompted, and click Enroll me in this course