Let's say we have a Blockchain consisting of 10 blocks, indexed from 0(Genesis Block) to 9(The last block).
If the data of a block, say block 5, is changed(maliciously), the data stored on the block, as well as subsequent blocks, becomes invalid.
What happens next?
It is impractical to drop the blockchain entirely. It is also impractical to recalculate all the hashes, in case of larger chains.
So, how is data of the current block, as well as that of other blocks, preserved in the event of an attack on a single block?