That is a very good question that is the subject of a lot of denial. You might want to read about the DAO hack and the Parity wallet hack. You will notice there are two different possible outcomes: in the first case, some of the Ethereum founders were major victims, and they used their influence to persuade a majority of the miners to implement a 'hard fork' to essentially roll back the blockchain. In the second case, the victims do not appear to be well-connected, and it seems their missing ~$30M of Eth is irretrievably lost (and it is not clear to me that a second hard fork would be feasible at this point, regardless of who the victims are.)