The initial signal
Search Console impressions cratered on a Tuesday. Crawl stats were normal. Rankings for head terms vanished.
The actual cause
A recent theme update injected a rogue noindex meta tag on paginated archive pages. 34% of ranking URLs were affected.
The fix and recovery
Removed the noindex, resubmitted sitemaps, requested indexing on top-20 URLs. Full recovery took 11 days. Monitoring caught it; most teams would have waited a quarter.