I recently had this Warning in my Event Log:
Event Type: Warning
Event Source: Windows SharePoint Services 3 Search
Event Category: Gatherer
Event ID: 2423
Time: 09:09:04 AM
The update cannot be started because all of the content sources were excluded by crawl rules,
or removed from the index configuration.
Context: Application 'Search index file on the search server', Catalog 'Search'
I checked the Search Administration, and found that all my content sources were "indexing" but no indexing were taking place. I could not pause the crawling, and nothing was being added to the Crawl Logs.
I then restarted the "Windows Services SharePoint Search" service, and the "Office SharePoint Server Search".
I could then pause the various Content Sources, and Indexing started.