The DAO was hacked by an unknown attacker who stole Ether worth around $50 million dollars at the time. After much debate, the Ethereum community voted and decided to retrieve the stolen funds by executing what’s known as a hard fork or a change in code.
Smart Contracts by definition aren't hackable. The "attacker" just found a loophole or unintended effect in their contract and used it. An Ethereum hack would be like, I dunno, finding an exploit in the implementation of the VM or something.
