I recall having an issue were the sandbox stop working. I was told to delete the sandbox and it will create a new the next time you start the course it work for me. Unfortunately, you lose whatever work you completed. Maybe someone else has a better idea, but this is an option. It worked for me.
Hello @mjaber995 and welcome to the Neo4j Community!
Are you saying that you were going through the course successfully and when you resumed, you received the bolt connection error?
Did anything change from one GraphAcademy session to another?
You might try deleting all cached pages and cookies as there may be some stale connection information.
Terminating the sandbox in sandbox.neo4j.com typically does the trick.
If you still continue to receive the bolt connection error, can you try opening a developer console in your browser to see if there are any errors raised?