Manual:Rebuildtextindex.php

rebuildtextindex.php is a maintenance>Special:MyLanguage/Manual:Maintenance scripts|maintenance script used to [re]create the text index that allows users to use the searching>Special:MyLanguage/Help:Searching|search facility in your wiki.

You should run it if you see that search gives no or few results.

This may take several hours, depending on the database size and server configuration.