Don’t just reset your search index in a production environment since this will also impact the analytics processing component (Read Reset the index in SharePoint Server 2013). Listed below is the syntax for the PowerShell command (the snippet below assumes that you only have one SearchServiceApplication)
The SearchServiceApplication.Reset method takes two parameters - public void Reset( bool disableAlerts, bool ignoreUnreachableServer) – I would recommend always setting disableAlerts to true if necessary. The value for the second parameter will depend on your specific case. If you also get a timeout when using the PowerShell cmdlet – you can use the steps outlined in SharePoint 2013 Content Index Reset Timeout – they worked for me.