Rebuild search index in Exchange 2013

The search index in Exchange 2013 can become corrupted and will need a re-seed (a re-index), you can check the status by running Get-MailboxDatabaseCopyStatus | FL Name,*Index*

To do this, stop the MS Exchange Search and MS Exchange Search Host Controller services:

Stop-Service MSExchangeFastSearch

Stop-Service HostControllerService

Then find the folder which contains the content index catalog, this is for each mailbox database individually, found at %ExchangeInstallPath\Mailbox\<name of mailbox database>_Catalog\<GUID>12.1.Single and rename it, for instance: C:\Program Files\Microsoft\Exchange Server\V15\Mailbox\Mailbox Database 0657134726_Catalog\F0627A72-9F1D-494A-839A-D7C915C279DB12.1.Single_OLD

Restart the services above:

Start-Service MSExchangeFastSearch

Start-Service HostControllerService

Then run Get-MailboxDatabaseCopyStatus | FL Name,*Index* to check the status (this need a couple of minutes to update).

Via: https://docs.microsoft.com/en-us/exchange/reseed-the-search-catalog-exchange-2013-help?redirectedfrom=MSDN#Anchor_2

If this doesn't work, also check this post: http://clintboessen.blogspot.com/2014/12/exchange-2013-fast-search-technology.html