Blockchain isn’t totally foolproof — there are a few ways that it can be hacked. “If 51% of the network of miners, or of work done by miners, is bribed to alter the ledger, this can result in corruption and incorrectness in the ledger.” Additionally, if one of the nodes of the blockchain network is disabled and it cannot communicate, it may be compromised
SOURCETalking about bitcoin’s blockchain; when a miner is trying to alter the ledger, other miners following the correct rule-see this and decline to accept the change. And as a result, the bad guy's work is pointless. At least 20% of the hashrate is now owned by publicly traded corporations, making it impossible for a miner with 51% of the hashrate to launch an attack quickly in a matter of seconds. Making this attempt can only result in troubles for the attacker, who will also face numerous legal actions. So launching an attack on the bitcoin blockchain is practically not feasible.