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
Are cookies enabled on your browser
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 clickÂ Enroll me in this course. Once you are enrolled in a course, it will appear under "My courses" any time that your are logged into that Moodle site.
If you are not already enrolled in a course you can search for it by name and description.