China has a long history of blocking or limiting access to websites, especially social networks and Western-owned sites. Wikipedia, Twitter, Facebook, Instagram, and many Google services are currently blocked in mainland China. WhatsApp has been partially-blocked in the past (for a while, video chats and some other functionality wasn't working), but now the entire service seems to be non-functional.
The new block is disrupting all aspects of the service, including normal text messaging. It's not hard to guess the reasoning for this move; WhatsApp is one of many services that support encrypted messaging, which by nature makes government surveillance difficult or impossible. The block has caused many Chinese users to switch to WeChat, a popular messaging service in the country. WeChat is owned by Tencent, a Chinese tech company that often accepts information requests by China's government.
Facebook did not respond to The New York Times for comment. This likely puts a dent in Facebook's plans to expand more services into the country, especially considering Facebook and Instagram were already blocked.
- The New York Times