1) Stop the following services:
Microsoft Exchange Search
Microsoft Exchange Search Host Controller
2) Locate the existing index directory folders.
They should be in the same path as the mailbox database with a GUID and word ‘Single’ at the end. With both services stopped, rename the GUID folder to GUID_OLD.
3) Restart the 2 services:
Microsoft Exchange Search
Microsoft Exchange Search Host Controller
The GUID folders should be recreated and reindexing of the databases started. It will take a fair amount of time for the rebuild. You can issue the following command to check the index status:
Get-MailboxDatabaseCopyStatus