Check that HMaster, HRegionServer, and QuorumPeerMain are all listed in the java processes. The error you are getting seems to indicate that the HMaster is not running. You can check this by logging in to the Sandbox and running the following command:
It is my guess that the QuorumPeerMain and HMaster will not be running. If not you will need to start Zookeeper and Hbase again.