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?
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. Once you are enrolled in a course, it will appear under "My courses" any time that your are logged into that Moodle site.
How do I jump between my courses?
'My Courses' block if it has been added to the page you are on.
Go back to the homepage (see below) and then use the main course block (if it has been added!)
Why is there no upload box?
This is either because:
The assignment has now closed.
The assignment is not yet open.
You already uploaded something and the settings prevent resubmissions.
How can I see my recent assignment feedback?
There are many ways you can access their feedback. The most common method is by simply going to the same place where you uploaded the work. Another common method is to follow the link in the recent activity block (if the teacher has included it on the course). Another method would be to access the grade book and then follow the link for the required assignment. Depending upon how the assignment was set up, you may receive an email when it has been marked with a direct link to the feedback.