in my opinion it's impossible for a blockchain to be hacked because it's decentralized (not controlled by any authority) and it's managed by it's users,
This solely depends on the protocol; being decentralized does not mean that when there is a means of hacking your system, you can't be hacked. There have already been several cases of different smart contract hacks (
Polygon ,
BSC , and others) in which the hackers penetrated the system, minting some altcoins from nowhere and pushing them into the market.
Like I said earlier in a similar post, anything that is being programmed is hackable; any little mistake from the developers can be used against them. For now, Bitcoin is the online Blockchain that I have not come across or read anything about being hacked before. I believe this is as a result of the developers, who are working very hard to upgrade and fix any bugs discovered in the system that could lead to it being hacked.