I Recently installed the "republished" October Cumulative Updates for SharePoint 2010. I then discovered that Search was not working. Crawl was going on forever and I was getting the message "Query Server not responding" therefore no propogation was happening . All my Search components were installed on a single server at this point. The Crawl component was also reporting a "Recovering" status. I added a new Query component and deleted the original one but this did not fix the problem.
I then remembered that I had seen something similar in SharePoint 2007 where updating the farm with a cumulative update would cause the Adobe PDF iFilter settings to be lost from the registry which in turn would break search. I have blogged this previously http://mrvsharepoint.blogspot.com/2009/03/moss-february-2009-cumulative-updates.html
It seems that this is still the case with SharePoint 2010 and certain Cumulative updates. I went back and added the PDF iFilter settings back into the registry (http://support.microsoft.com/kb/2293357), rebooted the server, did a full crawl and it all worked again.