Here is what I recommend. Not sure what course you are taking.
The Cypher Fundamentals course uses the movies sandbox.
The Importing CSV Data and GraphData Modeling courses use the blank sandbox.
All other courses use the recommendations sandbox.
Got to your account at sandbox.neo4j.com and terminate the appropriate sandbox.
Go back to GraphAcademy. It should recreate the sandbox a put it in a state that is ready for whatever challenge you are working on.
Let us know if that does not do the trick for you.
'Do you not see the $neo4j prompt in the right window?'
I can see the '$' without the 'neo4j'.
'Did something change in your environment (ad blocker, connect to a VPN?).'
Not. It just stopped while I was doing the challenge.
'Can you access the sandbox with Neo4j Browser in the sandbox.neo4j.com site (Open)?'
Very strange indeed.
You can execute the Cypher code in the Neo4j Browser window in sandbox.neo4j.com, but you must click the Validate/Verify button to check if your changes to the graph are correct.
If you could provide a Developer Console screenshot when you attempt the Challenge, that might better help us to troubleshoot the problem you are facing.