According to the official ZenCash blog, at approximately 2:43 UTC on June 3rd, ZenCash became the target of a 51% attack.
For the uninitiated, a 51% attack is when a malicious person or group takes control of a blockchain through obtaining over 50% of the hashrate, allowing for the hacker(s) to take control of the network.
However, ZenCash’s team was ready for a situation like this to occur.
The team quickly initiated an investigation on the hashrate distribution to confirm that it was, in fact, a 51% attack. While hash rates were being investigated, other members of the ZenCash team simultaneously contacted exchanges to increase confirmation times to help mitigate the risk of a double-spend.
Once the investigation was finished, the ZenCash team still found that there were two double-spent transactions. This meant that the hacker, whoever it was, was permitted to spend their 19,000 ZenCash two times, effectively doubling the worth of their ZenCash.
How Did This Happen?More here,
https://www.newsbtc.com/2018/06/04/another-cryptocurrency-experiences-51-attack-following-bitcoin-gold-verge/