The incorrect server server or shard has been queried (eg shard 0 does not exist on this server) or an incorrect Shard Id is being used.
No results (numFound="0")
If you are not getting any documents returned verify you have the correct Shard Id. If the page is still accessible in the UI, the data still exists. If you are receiving a 404 error in the UI, the document has been purged or does not exist.