I have had a look and I can't see anything wrong with Graph academy.
It could be that the sandbox that was created for you isn't responding.
If you goto sandbox.neo4j.com , log in with the same account, you should see the a ‘blank’ sandbox instance. If you terminate/delete the instance, GraphAcademy will recreate you a new one when you return to the course.
Sandboxes do expire after 3 days, so it may have already been deleted, depending on when you joined the course.