Due to a recent power outage at a client site, their Exchange search catalogs became corrupted. The issue at hand was that searches in Outlook or OWA only resulted in showing results prior to the outage.
To confirm that this was the case, I opened the trusty EMS and ran this command on an affected individual.
test-exchangesearch username | fl
This result confirmed the catalog corruption:
ResultFound : False
SearchTime : -1
The fix is rather easy, but depending on your database size, can be rather lengthy. The script below stops the Microsoft Exchange Search Indexer, deletes the Catalog of the database in question and restarts the Indexer which fires off a complete rebuild.
You can find the script here (from where it must be run): <drive>:\Program Files\Microsoft\Exchange Server\Scripts
Syntax: ResetSearchIndex.ps1 -force <database>
Example: ResetSearchIndex.ps1 -force "Mailbox One"
The rebuilding of our catalogs took almost an hour for a 80GB database. And that was on a fairly powerful system with 32GB of RAM.