Sorry. To answer your question, some cryptocurrencies, like bitcoin, as it currently stands, will completely bite the dust as soon as a quantum computer is available, for two reasons.
One is that it uses, fundamentally, digital signatures, which as such is not necessarily broken by quantum computing if it's implemented properly, but it uses specifically an RSA-type digital signature, if I'm not mistaken. That is broken with a quantum computer. The entire so-called blockchain will be broken as soon as a quantum computer is available, when the root is protected by RSA signatures. That's one thing.
The other thing is that cryptocurrencies are based on the notion of proof of work. They claim that in order to mine coins, you need to work for so much time. They claim there is a fundamental obligation to work for so much time or do so much work in order to mine a new coin. This is not true because quantum computing would allow us to mine coins much more efficiently, although only quadratically more efficiently. It's back to the NP-hard question. Still, quantum computers would break the basic assumption of the proof of work that is behind most cryptocurrencies.
This doesn't mean that cryptocurrencies are dead. It means the way they are currently implemented will completely bite the dust when a quantum computer becomes available.