Bing was unavailable for a short while on January 2, and it looks like a bad code update from Microsoft may have caused the outage. Citing an unnamed source, Reuters mentioned that Microsoft struggled to roll back the changes once the update was live, which led to the short interruption of its search services.
The outage knocked out Cortana's ability to query web results as the voice assistant's searches are powered by Bing. According to Reuters' source, Microsoft had to "shut down its groups of linked servers to get back the point where everything worked smoothly" after the initial roll-back of the update failed.
In an official statement, Microsoft acknowledged that its services were affected, and that the issue has been resolved:
"This morning, some of our customers experienced a brief, isolated services interruption which has now been resolved."
Yahoo (whose search is powered by Bing) was also affected by yesterday's outage.