You may have just upgraded or migrated your installation to another server. You try to search or reindex your site data and see an error stating Search is Unavailable.
ElasticSearch may be running but not connecting the way it should. To resolve this:
- Open Services on the web server
- Locate ElasticSearch
- Stop the service
- Start the service
- Reindex the site by going to Admin > Site Search and clicking re-index my site data
If that does not resolve the issue, verify the service has enough memory allocated - see the ElasticSearch Memory article.