There is a known issue where the next button sometimes appears deactivated (grayed out) at the end of the first section of a week/unit for some learners in some browsers. EdX currently does not have a resolution for this error and we do not know what causes it.
To navigate to any open section of the course, use the Course Outline. Click "Course" (next to the word Discussion) in the top menu and it will take you to the Course Outline. Scroll down to the week you want to access, and click the next section in the sequence.
To help edX solve this issue, please use the Help link on the top of the edX screen (or email info@edx.org). Tell edX your browser type, browser version, the course you are enrolled in, and the week/unit number/name where it occurred.