Recently a client of mine would receive this error when attempting to search within a SharePoint site collection.  This partcular issue was a bit more involved than I had originally expected.  Reason being, when I went to manage the configuration of the Service Application I received a error message stating there was an error loading the Service Application.  Ok, so at this point I decided to create a new Search Service Application.  This is an easy enough task.  Deleting the previous one proved complex (still working on it actually).  Well once created I would receive the error tagged to this post when attempting to search.  The fix was easy enough; I navigated to Manage Service Application Associations under Application Management, clicked the default proxy group associated with my main web application, and checked the box for my new service application.  I then started a full crawl.  The crawl recently finished, and now my search queries are returning results. Smile