Scenario:
Crawl History shows 0 successes and a bunch of security errors.
The following error is logged in the Error Breakdown:
“Access is denied. Verify that either the Default Content Access Account has access to this repository, or add a crawl rule to crawl this repository.”
Possible Solution:
Check your hosts file. In my case, host headers had been commented out for testing and never changed back.
SharePoint Mustard 