According to a report from Bloomberg, Bing is now accessible in China once again, and that it might have been an actual technical error rather than an attempt to censor the website. Given that Microsoft for the most part has played nice and complied with the various regulations and laws in China, it does seem strange that Bing would have suddenly been censored.
It also seems that quite a few users in China are pleased to see Bing return, with many posting on Weibo celebrating the return of the search engine. To date Microsoft is one of the few tech firms from the west whose services are allowed to be used in the country. Google had previously pulled out of China, although there were rumors that they were planning a return (Google has since denied this).
Other websites and services such Facebook, YouTube, and Twitter are also banned in the country, at least officially because it seems that many have found their way around it through VPN services.