For us to identify why a user is experiencing issues, we need to gather information about their user environment.
If someone is unable to open their course, could they return to their homepage and make a screen grab of their 'HOW'S YOUR SYSTEM?' block. When we see this, we can better advise you.
Course licence has expired
If your users can log in but not see their course on the Homepage, the reason may be due to the course licencing expiring. You can check this on the course's 'Enrolled users' page:
