For hackers it is virtually impossible to bring down a blockchain unless the source code is badly written. However if 51% of the witnesses collaborate (or one witness gets 51% of the power) to attack/tamper with the blockchain there will be serious problems. Does this mean that the blockchain is down then? No, but it won't be very useful any more because the 51% attack could steal all funds from the users. Is this likely to happen? No, because the steemblockchain uses delegated proof of stake so they can loose their stake (which is 51%) when caught with tampering. If the source code of the blockchain is badly written and expose a backdoor to attack a witness (or all witnesses at once!) a single hacker could tamper with the data (move funds etc) what will make the blockchain die eventually unless the changes made by the hackers are reverted (aka what ethereum did after the DAO attack).