Sometimes I feel as if computers get emotional. I mean, they get angry and start messing with your work. Also, they start breaking things that weren’t supposed to break. This week, my computer decided to break one of the Solr indexes used by a Sitecore 9 installation.
I got into the same situation as this folk here.
From what I could find, it seems to be a bad error message due to a corrupted index. The index corruption might have been caused by a failure during the computer restart process. As I recall, I restarted my computer due to a required Windows update.
Solr 6.6.1 has a known lock bug and it was fixed on version 6.6.2 – https://archive.apache.org/dist/lucene/solr/6.6.2/changes/Changes.html#v6.6.2.bug_fixes.