Which version of the HDP Sandbox are you running? (2.4 or 2.3.x)
There is a chance that zeppelin can't connect to the Spark interpreter, so from the Interpreter page (shown), click on the [restart] button, wait for 5 seconds, and go back and refresh the notebook page. Also, from the notebook page in zeppelin, click on the gears icon to make sure Spark is one of the listed interpreters.
i'm running HDP 2.4 , i restarted Spark and it appear in the list of my notebook's interpreter but the problem still exists !