The Solr crawl db may be corrupt if you see the following:

• Clicking on Node Status gives an error 'ManifoldCF cannot communicate to the Solr crawl maintenance database..."
• HSQL java.exe process is missing from Process Explorer on the Solr server.
• ManifoldCF web page throws an error while navigating to Status and Job Management.
• New content isn't being indexed.
• The crawl doesn't appear to end.

Follow these articles to fix the reported behavior. Note that search is down while the crawldb is being rebuilt.

Clearing the Solr Crawl Database

Maintenance on Solr Crawl Database