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.