Blockchains and cryptocurrencies use public key cryptography under their hoods.
Most experts agree that quantum computing poses a great threat to most cryptographic principles such as ECDSA algorithm used by Bitcoin & Ethereum.
Quantum Computing research is making progress and could become a reality in the coming years-
See this Is it time for the blockchain & cryptographic community in general to explore possible ways to mitigate Quantum computing and develop the so-called quantum-resistant or post-quantum cryptography?
