Please follow below tips to troubleshoot the issue.
1.Go to Central Administration -> Manage web applications -> Select the web application you use -> Services Connections -> Make sure the Search Service Application is selected.
2.Go to Central Administration -> Manage service applications -> Make sure the Search Service Application and proxy are started.
3.Go to Central Administration -> Search Administration page -> Check Administrative status and Search Application Topology are running.
4.Go to Central Administration -> Manage services in this farm -> Make sure Search Administration Web Service, Search Host Controller Service, Search Query and Site Settings Service and SharePoint Server Search are compliant.
5.Go to Central Administration -> Manage services on server -> Make sure Search Administration Web Service, Search Host Controller Service, Search Query and Site Settings Service and SharePoint Server Search are started
6.Go to windows services -> SharePoint Search Host Controller and SharePoint Server Search 16 are running.
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