This article explains why some courses are grayed out.

Whenever you see a grayed out course in the My Courses widget, it means you are enrolled in a course that has not yet started. You are unable to click on the course or open the course.

The course will automatically open as soon as the start date has passed. In case the course is still grayed out when your first lectures have started, check with your instructor if this is indeed correct or ask if he/she can open the course.