I notice it isn't showing any new results, specifically posts or comments made in the past 24 hours[1] ("We found no items matching your query for this period").
Algolia seems to have had an "incident" earlier today[2], I wonder if the two are related.
[1]: https://hn.algolia.com/?dateRange=last24h&page=0&prefix=false&query=&sort=byDate&type=all
[2]: https://status.algolia.com/