ContentIndexErrorMessage : The Microsoft Exchange Search Host Controller Service is not running on server

After updating Exchange 2016 CU16 to CU19 and applying the "HAFNIUM" security patches, some databases were in Failed state. 

Error 1:

in ECP was getting the error "Content index state: Failed" against the database

Error 2:

in Exchange Shell, running 

Get-MailboxDatabase DatabaseName | Get-MailboxDatabaseCopyStatus | ft Name,*index* 

was getting this error

"ContentIndexErrorMessage     : The Microsoft Exchange Search Host Controller Service is not running on server"

Solution:

After checking the "Microsoft Exchange Search Host Controller" Service on the effected server, found that this "Microsoft Exchange Search Host Controller" was in stopped/disabled state. 

after changing the startup type to automatic of "Microsoft Exchange Search Host Controller" service and starting it and restart the server.

the issue was resolved.