Have there been instances of blockchains been hacked?

[JLCoin]

If so, why are people so confidant that it won;t happen again? Thanks!

[VitKoyn]

If you are referring to the Bitcoin blockchain the answer is no. No one has ever succesfully hacked the blockchain of Bitcoin. To be able to hack Bitcoin blockchain you need to have 51% of hash power of Bitcoin network which will cost you millions of dollars and impossible to achieve due to the huge community of Bitcoin miners. You can also hack blockchain without having the majority of computing power if you can solve the Bitcoin algorithm SHA256 in reverse which is very hard to solve.