Well, as much as I'd like to blame Nick, it's not just his fault. In preparation for our upgrade to Elastic v2.4.6 (on our way to v5.5.2) we started re-indexing all sites in the network on Friday and I kicked off Stack Overflow on Saturday morning. Stack Overflow is a pretty large site to index (46m documents) and it takes about 3 hours at full speed. Indexing that many documents while serving live search traffic hurts the cluster so we decided to introduce an abort function in case we needed to stop it in a hurry.
But... that new functionality had a bug and the deltas that run to keep the indexes in sync with their sites didn't run. I've now fixed the bug and pushed to production. Indexes are catching up now so all should be well in the next hour or two.
Thanks for the heads up!