The classical computers process data while the quantum computing is a new method to process data and solving problems that classical computing are not capable of.
There are some computations that was thought can not be possible but now becoming a matter of time by quantum computer.

As I read it, it was written that quantum computing that computer giants like google are working on now can not break blockchain and the underlying cryptography. But, with time, there can be more sophisticated ones that can be a threat to blockchain. So, they are a threat to blockchain and cryptocurrencies.

It will be better to create a quantum resistant layer to an existing blockchain protocol to increase its security or create a quantum resistant blockchain from scratch. They are the recent approaches.