If you can see the title of a lesson on your Course Page but don't see the start/resume button, look to the left of the lesson title. There is a small arrow/triangle. If it is pointing down, the button will be hidden. Click the arrow/triangle to expand the section. You will now be able to start/resume the lesson.
If you cannot see any of the start/resume buttons, you may need to logout and login again. Make sure you have logged in with the correct username and password for this course.