Summary of report from user on mailing list:
If zookeeper is down or doesn't have quorum when you start Solr nodes, they will not function correctly, even if you later start zookeeper. While zookeeper is down, the log shows connection failures as expected. When zookeeper comes back, the log shows:
INFO - 2013-08-09 15:48:41.528; org.apache.solr.common.cloud.ConnectionManager; Client->ZooKeeper status change trigger but we are already closed
At that point, Solr (admin UI and all other functions) does not work, and won't work until it is restarted.