My Client upgraded their SharePoint from 2013 to 2016 and from a standalone server to a 2-tier environment. (1 x DB server + 1 x App/Web server) The Search did work for a while, but now it stopped working. It does the index and crawling, but when searching there is no results. The crawl log shows 72k errors, much more than the 1k Successes. In the 'Crawled Properties' and managed Properties windows, it does not show any managed property entry and only has 'An unknown error occurred' in red at the top of the screen. Opening 'Launch query builder' also gives: Error: Not able to connect to search service to retrieve valid settings'. Can I delete the Search Service Application and rebuild it?
Take the back up of all search service application databases - mostly 3 to 4 database will be there, you will get these details from the search service administration page of central admin... then click on the index reset link from the crawling section. Then, restart the search service from Windows service and run the full crawl.
If it still does not fix the issue, you may re-create the service application.
Using the below PowerShell also search service can be reset:
For details steps, you may refer to the below article:
You could go to search administration page to check the status of six search components and whether any error message in the page. In addition, you could check ULS logs for more details about error. Investigating logs will help to understand the problem.
Of course you could choose delete the Search Service Application then rebuild it.
I've deleted the current Search Service Application (made sure the app pool is also deleted) and recreated the SSA. Everything is working as it should.
The error I got when opening 'Launch query builder' also gives: Error: Not able to connect to search service to retrieve valid settings' - it pointed to the previous server, that is why it gave an error message.